I just put in a custom PROM, and don't know for sure if this problem I have is related to that. My car is stumbling badly from idle through about 2000 rpm. It's almost a miss. The tach is erratic as well. It appears to happen once I'm in closed loop. Sometimes it's to the point of a near stall with almost no acceleration. I basically have to nurse it up over 2000. At that point it takes off and actually pulls pretty hard. I just don't know if this issue is chip related or a coincidence. The IAC is a brand new Delco and my TPS checks out. I set the IAC up prior to the new chip and the car drove fine. It idles okay, and there are no codes. I have heard a muffled backfire the odd time as well.

 

Looks like it's doing it on the factory MEMCAL as well, although not near as bad. The idle on the chip is more lumpy as well. But once it's under way the performance increase is noticeable. Just not sure where I should be looking.I have Bosch III 24 pound injectors and I know the chip has those accounted for, so I'm wondering if fuel pressure is a possibility.