What would possibly be the cause of an Idle problem?

What is your question?

I had my car not wanting to start. Push gas pedal would stall out after took foot off pedal. Changed dash pot with one that works. Just cant seem to get the idle to be consistent. Today if cranked but idled so low it almost cut off turned on the air it sped up. Turned it off went back down. Put in reverse almost shut off put in drive almost shut off. Drove down the street is perfect. So do I have a bad sensor or sticking throttle. My engine light will come on and then go off. Guess I need to get sensor test when that light comes on. Hard to find anyone with a obdll diagnostic reader for older cars. Heard Advanced Auto parts or Discount parts can read the error codes and ususally do for free. Any help would be great with this issue. My car is very sound. Had no issues other than things being old. Keep the oil changed every 3 thousand miles and all the belts and things that just ware out replace. Thank you for any help!