98 cadillac deville shuts off at idle sitting at stop light sometimes

Try filling it with a top grade premium gas.

Check for loose vaccum hoses. If everything looks in place, I would suggest you collect as much information as possible: Engine light on? what is the troublecode?

Depending on the trouble codes, it may be the ECM. (computer)
You can have the car scanned for codes or attempt it yourself.
The "set timing mode" is the section of the computer program that you access to change the base timing. It should not show up on the display unless you access it.

Sticking throttle plate maybe , clean with carb. cleaner .
2002 Saturn high idle at red light
007 Idle Speed Problems Scan Data for Idle

sounds like it and could very well be as some new cars a/c works off vacumm too

It could be a bad or weak fuel pump, bad or loose wires, are there any Check Engine or Service Engine Soon lights staying on your dash?

Plugged fuel filter or bad pump

Air Idle Motor or Idle Solenoid

97 cadillac deville digitil dash shut off????????cheched fuses?

idle speed control .its idling too slow.

sounds like a 12V drain on the system, like a trunk or glove box light that won't shut off.> Could be many. many other reasons why. The way I would find the draining circuit would be to remove the neg cable of the battery. I then would connect a 12V test light between the neg battery TERMINAL and the neg battery CABLE. With a 12V drain the light will shine. I would install into the light a 9V battery powered recepticle to hold computer and radio station memory. Then I remove fuses 1 at a time, then reinstall, at the same time looking for the 12V light to go out. Hope this helps.