Hi guys. As title says. Have a 383 HSR motor. My problem is when I start the car it idles fine. Its a little rich first thing in the morning but after that it seems ok. Anyway after I drive it for a bit the idle likes to jump up and stay up. When Its normal its about 800-850 and when it jumps up it sits around 1200-1300. Sometimes if I shut it down then restart it it will go back to normal but then after more highway driving it kicks back up. The weird thing is it doesn't seem to happen if I'm just driving around town below highway speeds. Anyhow if anyone has any ideas at what I should be looking at that would be amazing. Also my iac and tps are new. Its a 91 speed density car as well if that makes a difference. Appreciate it guys. Thanks!

Cam specs?
Has the ECM been custom tuned for the mods ?
Any codes showing?
Done any datalogging to see what is happening ?

ECM was tuned yes. No codes. Was a code 42 but got that cleared up before this started happening.

Same issue here myself. Used to be a blip of the throttle and it would idle down (now a blip doesn't bring it back down) have aswell put in a new TPS(twice) and changed out the iac with another but hasn't solved the issue. Just a slight pet peeve in an otherwise loved setup. Will continue trying to track down the issue and if the culprit is found i'll post up.

looking at the data real time would be ur best bet..i think it might be worth a shot to grab the tps wiring and move it back and forth with your hand as this is happening and see if that does anything...u sure u have no vacuum leaks?