Dryer will not shut off in any 'auto' mode. Time display ticks down, then jumps up again and keeps running. Dryer does works properly when 'timed dry' is selected. Checked and cleaned exhaust vent. Checked thermistor for proper resistance value (OK). Checked and cleaned sensor bar, measured resistance (OK). Please help!

check for any loose connections or faulty components

will it shut off if you run it on time only mode?

In some dryer's the control panel relies on a thermistor rather than a CYCLING thermostat to regulate the drum's air temperature by monitoring the component's resistance changes; resistance goes down as temperature increases and up when temperature decreases. Once the drum's air temperature reaches a certain level required to dry clothes, the control panel shuts off the heater. The panel will turn the heater on again and begin another heating cycle when the thermistor indicates that more heat is needed to keep the air temperature constant inside the drum

Lastly check your moister sensor. ( located inside the dryer door usually) Especially if machine seems to shut down early and clothes are still wet.
Test with a meter at room temperature and it should show continuity.
A failed moisture sensor will affect the dryer run time in the automatic moisture sensing cycle but it will not affect the heating of the dryer or the timed cycle. Which are reflected by the thermostats.

Read more :http://www.ehow.com/info_12203962_check-dryer-thermistor.html

high speed is an aux and low speed should be another aux if to run all day designated program to each aux

you could have bad gas coils.if the timer doesn't advance in auto dry most likely you're having heat problems,when the dryer heats and then the heat shuts off then the timer will advance,when the heat comes back on the timer stops,it does this until the clothes are dry.gas coils when starting to fail will work normally until they get hot and then no heat until the coils cool down then they will work again,on time dry the timer just keeps counting down until the time runs out no matter what's happening to the dryer.i've seen the coils fail after the first 10 minutes and other times for 20 to 30 minutes but usually if the load is big and in time dry the clothes will feel warm and you'll need to dry them a second time to get them fully dry,also while you're in ther you should clean out the dryer at least once a year,clean out the duct that the lint filter slides into,vacuum out the cabinet where the motor is and blow out the vent line,if it's a long run i use a leaf blower to blow out the line,remove it from the back of the dryer,stick the leaf blower into the vent line and blow it out,i've blown out birds nest out of some lines, your clothes will dry on time,you won't go through so many parts and you won't have to worry about a dryer fire,the gas coils are on top of the gas valve,remove the two phillips screws,lift off the cover and change out the coils.next time you use the dryer keep an eye on it,after it runs for 20 minutws start checking it,you'll hear a click,then another click and the flame will lite,look into the gas tube and if the flame isn't liting then you have bad gas coils.

• Start tapping the F8 key. The Windows Advanced Options Menu appears. If you begin tapping the F8 key too soon, some computers display a "keyboard error" message. To resolve this, restart the computer and try again.
• Ensure that the Safe mode option is selected.
• Press Enter. The computer then begins to start in Safe mode.
• When you are finished with all troubleshooting, close all programs and restart the computer as you normally would.

If this does not work you will probly have to go back to factory settings

If the timer is not advancing but is ticking that could be the problem. A dryer achieves a set temperature and stays there for a certain amount of time during a cycle. If the time never changes it may just keep heating up until the heat sensor shuts it down. If it is starting and shutting down its likely still related to the heat sensor.

Auto sensor is not working. Replace it.

Faulty selector switch or control card. There is the possibility of a faulty moisture sensor. Can you operate the machine in manual mode ?