I got a 87 5.0 mustang, anyways after i warm up my engine, it sometimes doesn't want to idle, it will just die, and then when i hold the gas down for a bit to keep up the rpms up for like 10 secs it will idle. Then it will be okay and then will randomly do it again. I have no idea what it could be. I'll be driving on the freeway then pull off a exit and push in my clutch and the rpms will drop till it dies, then start it up again, does it again then it will idle up and down for a bit find its idle and hold it. No idea what it is.

 

tfi module more than likely. could be the coil too, but coils dont generally fail intermittently. tfi modules do though.

 

http://www.muscularmustangs.com/iac.php
check that out maybe it will help, it helped me...