My 1989 f150 will start, go to about 1100 RPM's then drop to 500 or so. It continues to bounce back in forth 4-5 times before settle down at 900 RPM's at idle. This problem only occurs after I've driven the truck (never happens when motor is cold). It's never died on my, but I feel like it's close every time. Can anyone tell me what is causing this?

 

I would suspect the IAC is the issue.

 

I would pull out the control module (computer) and look for any burned out resistors. Mine was doing the same thing. I replaced the IAC and nothing changed. Pulled out the computer and found a burned resistor. Replaced the computer and I haven’t had any idling issues since.