My 91 Formula I just purchased has a really bad idle when the car warms up, it goes from 500 rpm to 1000 rpm and back and forth. When the cruise control is set it seems to jump around at this time as well as when ever your just holding the pedal at a constant speed. I replaced the IAC and another small sensor located just under the IAC, and neither did the job. As a matter of fact now when the car warms up and is restarted it idles at 2000 rpm's and slowly comes back to normal. This car has never been tuned up or had any modifications what so ever. It runs rather well for 95K, the previous owner did absolutely nothing other than oil changes. ANY HELP is greatly and always appreicated. Thank You Very Much !!!

 

Time for a full blown tune-up and check everything out. If nothing has been done and the ca has 95k miles it probably could use new injectors, distributer cap rotor etc etc...

 

A idle that goes up and down alot can be a bunch of things.Lets see what I can think of here thats most common.

Vacuum leaks:worn intake gaskets,TPI gaskets,broken vacuum lines,etc.

Dirty TB:needs re cleaning,remove carbon coking

TPS/minimum air speed out of ajdustment.

IAC not properly resetted

Fuel Pressure is low,restricted fuel filter or weak pump.

These are the basics I would check into first..a high idle and hunting idle is almost always caused by vacuum leaks.

Check everything visual,all vacuum lines and hoses..and with the car running,spray a little carb cleaner in various places a short spray at a time,if the idle immediately changes,you probably found a leak.
Hope this helps gets ya started somewhere.