(I posted this in the V6 forum also so hopefully I'll get at least one answer)

So I have a 1:30 break between my classes and i usually sit in the library and study or stare out the window but today I needed to get some books from my car and I decided to sit there and listen to some music. I sat in there for about 10 minutes with the engine running since it's still 100 degrees plus here in TX. I noticed after about 4 minutes that my enigne was jumping aroung between 900 and 1200 RPM sometimes jumping as high as 1500 for a few seconds. Any suggestions? My initial thought is IAC sensor but I wanted to check and see if anyone else experienced anything similar

 

What year? You mentioned an IAC so I'm assuming older than 95/96. If you're just idling, then a vacuum leak could be the culprit also as it switches from the (cold) closed loop to (warm) open loop on the emissions cycle. Open loop on older stangs relys on vacuum lines as well as IAC, O2 sensors and a few other items.

 

2000 6er w/ auto

 

It could be the TPS is getting a worn spot in the carbon film that modifies the signal from the ECU.It works like the turns of wire in a rheostat but it's more sensitive.It could also be the IAC valve needs cleaning.