Why?
 

Why do mustangs sometimes rev up and down at idle? The mustang I have now usually idles normal when I push in the clutch at a stop sign, but sometimes when I push in the clutch the rpms will drop almost to where the car dies. Then the rpms will shoot up real quick and then the idle starts going up and down. I hate being next to someone at a stop light and having my car doing that. I had an 88gt that did the same thing after I converted it to mass air. Does it have something to do with the cars' computer? Is there any way of fixing it?

Say hello to the infamous "rolling" or "surging" idle, the "reason" this happens can be many, many things, from simply a bad "sensor", to a poorly matched engine package. The best thing to do is run an EEC test, to see if you have any bad sensors and such, make sure the car is in top tune, etc.

Make sure you don't have any vacuum leaks too.

Would like to add this too, on one of my 5.0's I had, it was a basic GT-40 type package and when i had underdrives on it it would surge, if i put the stock pullys back on it wouldn't, weird huh?