This is an 83 with a fuel injected 350. When i

well if the van is an 1983 it does not have one , as gm did not go to throttle body fuel injection till 1987 , no what u have there in the photo is a good old fashioned carb , same thing that still works today and works well too , but nowadays everyone is removing them to add fuel injection on there vintage rides

only time ive ever seen a carb maybe not perform well was at 6800 feet in elevation pulling a trailer

It sounds like you have a restricted fuel flow, which could possibly be a weak fuel pump or a bad fuel pressure regulator. Do a fuel pressure test to see if your fuel is being restricted. If it's only in the morning, that means its loosing its pressure over night. That could also be caused by a bad fuel injector. Test your pressure first.

Sounds like the fuel pump is going bad(or in this case,has gone bad). I suggest that you change the fuel pump and use a fuel cleaner in your gas tank as dirty gas may have caused the problem to begin with.

I doubt it is ignition related.
Could be a problem with the idle air control valve or a dirty throttle body.
Could also be a vac leak, or fuel pressure issues.

If all plugs ,wires etc have been done, next i would confirm proper fuel pressure, then I would check throttle position sensor. An old analog ( with sweeping arm) multimeter on ohms, remove connector at TPS and connect to 2 of the pins on TPS .(throttle position sensor) Engine off, rotate throttle, or have someone push it from inside, slowly, gradually and needle of multimeter should sweep across as well, if not try different combination of 2 pins. When you find right combination that makes needle move when throttle pushed, observe the needle as someone pushes throttle slowly and steady from slow to wide open. If the needle jumps, or falls off and comes back up as throttle steadily pushed, then replace the TPS. If the needle slow and steady moves up /across and no stumbling, the TPS is probably ok. Make sure you test it a few times at least.

APP sensor went to limp home mode. The APP or throttle control uses to angle sensor when one sensor does not report it goes to limp home mode.The ECM cross checks the throttle angle so one is off. The late module assembly that is used is mounted to the floorand was more damage from spilled drinks. some where water damage the majority where coffe damage.go to the floor pedal position gain acces to the connector the inside connector may be contaminated,if you decide to clean it youll need a electrical cleaner clean the inside connectors, with the engine off.

Sounds like the carburetor needs adjusted. As I am guessing it is not fuel injected.

Depends on the 8th digit of your VIN code = engine size.

VIN "6" - 8cyl. 6.0L 4BL (4 barrel carbureted)
VIN "8" - 8cyl. 4.1L TBI (Throttle Body fuel Injected) (95 % of 83 Cadillac Fleetwoods)
VIN "9" - 8cyl. 6.0L TBI (2% of 83 Cadillac Fleetwoods)
VIN "N" - 8cyl. 5.7L MFI (Multi-port Fuel Injected) Diesel OHV (Overhead Valve)
(3% of 83 Cadillac Fleetwoods)

well want to help is plowing any smoke whit or black if so let me know witch some time a bad thermostat can do this it will mess with sensor it could be fuel psi have you checked or replaced all filters is this truck bio fuel if so the line can Cristal up a little sea foam can help can find at local parts store put it in the tank and will clean fuel injectors and lines hope this helps make sh ere air filter is clean and some time throttle position sensor can do this hope this helps

check the fuel perasure regulator using a hand vacuum pump if it holds preasure check fuel hold preasure and flow preasure