Thermostat went up to HOT, then over a 40 minute period acceleration died to about 40 MPH and could not go over a hill. when engine turned off, looked under hood to find engine was not hot at all.

You can try a drive cycle, see if it helps? There may be an idle relearn, if just idle issue? Usually it will relearn idle on its own by driving.
Drive Cycle Recommendations:

1. Most OBDII monitors will complete more readily using a "steady foot" driving style during cruise or acceleration modes. Operating the throttle in a "smooth" fashion will minimize the time required for monitor completion.
2. Fuel tank level should be between 1/2 and 3/4 fill with 3/4 fill being the most desirable.
3. The Evaporative Monitor can only operate during the first 30 minutes of engine operation. When executing the procedure for this monitor, stay in part throttle mode and drive in a smooth fashion to minimize "fuel slosh".

WARNING
STRICT OBSERVANCE OF POSTED SPEED LIMITS AND ATTENTION TO DRIVING CONDITIONS ARE MANDATORY WHEN PROCEEDING THROUGH THE FOLLOWING DRIVE CYCLES.
For best results, follow each of the following steps as accurately as possible:
OBDII Monitor
Exercised Drive Cycle Procedure
Purpose of
Drive Cycle Procedure Drive Cycle
Preparation 1. Install scan tool. Turn key on with the engine off. Cycle key off, then on. Select appropriate Vehicle & Engine qualifier. Clear all DTC's/ Perform a PCM Reset. Bypasses engine soak timer. Resets OBDII Monitor status. 2. Begin to monitor the following PIDs: ECT, EVAPDC, FLI (if available) and TP MODE.
Start vehicle WITHOUT returning to Key Off. 3. Idle vehicle for 15 seconds. Drive at 64 Km/h (40 MPH) until ECT is at least 76.7°C (170° F). Prep for Monitor Entry 4. Is IAT within 4.4 to 37.8°C (40 to 100° F)? If Not, complete the following steps but, note that step 14 will be required to "bypass " the Evap monitor and clear the P1000. Engine warm-up and provide IAT input to the PCM. HEGO 5. Cruise at 64 Km/h (40 MPH) for up to 4 minutes. Executes the HEGO monitor. EVAP 6. Cruise at 72 to 104 Km/h (45 to 65 MPH) for 10 minutes (avoid sharp turns and hills) Note, to initiate the monitor: TP MODE should =PT, EVAPDC must be >75%, and FLI must be between 15 and 85% Executes the EVAP Monitor (If IAT is within 4.4 to 37.8° (40 to 100°F)) Catalyst 7. Drive in stop and go traffic conditions. Include five different constant cruise speeds, ranging from 40 to 72 Km/h (25 to 45 MPH) over a 10 minute period. Executes the Catalyst Monitor. EGR 8. From a stop, accelerate to 72 Km/h (45 MPH) at 1/2 to 3/4 throttle. Repeat 3 times. Executes the EGR Monitor. SEC AIR/CCM (Engine) 9. Bring the vehicle to a stop. Idle with transmission in drive (neutral for M/T) for 2 minutes. Executes the ISC portion of the CCM. CCM (Trans) 10. For M/T, accelerate from 0 to 80 Km/h (o to 50 MPH), continue to step 11. For A/T, from a stop and in overdrive, moderately accelerate to 80 Km/h (50 MPH) and cruise for at least 15 seconds. Stop vehicle and repeat without overdrive to 64 Km/h (40 MPH) cruising for at least 30 seconds. While at 64 Km/h (40 MPH) , activate overdrive and accelerate to 80 Km/h (50 MPH) and cruise for at least 15 seconds. Stop for at least 20 seconds and repeat step 10 five times. Executes the transmission portion of the CCM. Misfire & Fuel Monitors 11. From a stop, accelerate to 104 Km/h (65 MPH). Decelerate at closed throttle until 64 Km/h (40 MPH) (no brakes). Repeat this 3 times. Allows learning for the misfire monitor. Readiness Check 12. Access the ON-Board System Readiness (OBDII monitor status) function on the scan tool. Determine whether all non-continuous monitors have completed. If not, go to step 13. Determines if any monitor has not completed. Pending Code Check and Evap Monitor "Bypass" Check 13. With the scan tool, check for pending codes. Conduct normal repair procedures for any pending code concern. Otherwise, rerun any incomplete monitor.
Note: if the EVAP monitor is not complete AND IAT was out of the 4.4 to 37.8° C (40 to 100° F) temperature range in step #4, or the altitude is over 2438 m. (8000 ft.), the Evap "bypass" procedure must be followed.Proceed to step 14. Determines if a pending code is preventing the clearing of P1000. Evap Monitor "Bypass" 14. Park vehicle for a minimum of 8 hours. Repeat steps 2 through 12. DO NOT REPEAT STEP 1. Allow the "bypass" counter to increment to two.
© Ford Motor Company

Check Mass Air Flow sensor.

Surges under load but only while going uphill Page1 - Chevy High ...

forums.superchevy.com > ... > General Chevy Technical Discussion

Mar 6, 2011 - 14 posts - ‎8 authors
Trying to help out a friend. ... The truck starts, runs, and drives fine until you get to a hill. When going up hill it begins to surge, almost stalling but not quite. ... rubber fuel lines suck shut under a heavy load and act like that. .... tranny fluid level o.k.? ya got me stumped. runs great when it's on a flat surface?

Ford 460 engine - RV mechanic

www.rvmechanic.com > Discussion Forum > Mechanic's Corner - Engines

Will not take gas going up a hill but will maintain speed at 55 mph. ... Want to do a diagnostic test on my 1997 F250 with 460 engine. ... only get to about 40 miles an hour if i push it the engine acts like its running out of gas,thank you .... The truck hesitates on acceleration, while driving on flat surface is fine, however climbing ...

rough idle, stalling, loss of power. - CarGurus

It sounds like the catalytic converter could be clogged up.

Next time the engine light comes on, get the codes pulled and get back to me

Sounds like a fuel supply problem. Does it pop through the carberator as it it is dying, like a lean back fire? If you drove it up a hill would it die before 35 or 40 mph? If the fuel demand excedes the fuel supply the engine will die, but would restart after cranking the engine for awhile. Possibly a plugged fuel filter, fuel line, rotten fuel hose, ( sucking air ) or tired fuel pump. Hope this helps.

it is not how many miles but the type of driving.

here is a OBD2 drive cycle test.

Generic OBD II "Drive Cycle" Most OBD II ("On-Board Diagnostics II") diagnostic monitors will run at some time during normal operation of the vehicle.
However, to satisy all of the different Trip enable criteria and run all of the OBD II diagnostic monitors, the vehicle must be driven under a variety of conditions. The following drive cycle will (theoretically) allow all monitors to run on (??) vehicle. (Note: Drive cycle specifics vary by vehicle!)

1. Ensure that the fuel tank is between 1/4 and 3/4 full.
2. Start cold (below 86°F /30°C) and warm up until engine coolant temperature is at least 160° F (typically requires at least one minute; up to 3 minutes).
3. Accelerate to 40-55 MPH at 25% throttle and maintain speed for five minutes.
4. Decelerate without using the brake (coast down) to 20 MPH or less, then stop the vehicle. Allow the engine to idle for 10 seconds, turn the key off, and wait one minute.
5. Restart and accelerate to 40-55 MPH at 25% throttle and maintain speed for two minutes.
6. Decelerate with using the brake [or the clutch!] by coasting down to 20 MPH or less, then stop the vehicle. Allow the engine to idle for 10 seconds, turn the key off, and wait one minute.

THERE IS A TEMPERATURE SENSOR ON MOTOR WHICH CAUSES THIS PROBLEM... USUALLY IT SHOWS A NEGATIVE TEMP READING WHICH CONFUSES THE COMPUTER CAUSING IT TO REV UP....HAVE THE ENGINE DIAGNOSED ON A SCANNER....GOOD LUCK...

this sounds like 1 of 2 possible problems.
a bad headgasket or a clogged catalytic convertor. i would lean more towards the convertor