I replaced my engine with a used one that seems to run fine, but after it warms up (10 - 15 minutes) the engine starts to surge from 300 to 1200 RPM then dies. I put in a new Delphi fuel pump as part of the engine change out (it was bad), swapped throttle body, MAP sensor, bit still no change. It idles perfect when it's cold, and even after the thermostat opens and closes a few times, but when it gets up to full operating temp the surging starts. Any help is appreciated.

1999 Grand Prix 3.1

 

you don't suppose it's the idle air valve, do you? How to tell: unplug it, (it's located by your MAF sensor) and then see if the problem persists. If the revving issue goes away, then there's your answer. But, unplugging it causes other issues--hard starting, etc. They're about $75 or so at Azone
abe

 

sounds like you have a bad throttle position sensor it is on the throttle body