More than likely it is your IAC....the little silver cylinder that is on the side of the T/B, take it off and clean it out with some throttle body cleaner and after it dries, pop it back on.

Hope this helps...keep us posted.

 

Gentleman their is actually a proper procedure to setting these engines up.
you must have the iac set properly and then adjust your t.p sensor once this fails then and only then you can start condeming parts. This procedure will also help diag the problem.

 

I assumed that this had been done already (and you know what they say about someone who assume(s), then cleaning up coroded pieces like those black and white connectors that prolly haven't been opened in years and years...

 

http://www.muscularmustangs.com/maintain.php