MercO5

My 1999 Mustang GT starts to sputter at low rpms when heated up but when I go above 2.1k RPM it runs perfectly fine and under WOT you can feel a small amount of hesitation and then it goes full power flawlessly. I have tried everything I could. I have replaced spark plugs, inspected COP, got a brand new MAF, cleaned throttle body and air filter, and had it professionally scanned for misfires or anything and nothing at all came up. I have no idea what it could be and it is extremely annoying . My question is, is this problem common and do any of you know for sure what it could be.

JJ Glo

Could be the IAC valve going bad and not being able to properly open and close to maintain a steady RPM.

My car started doing that before the IAC completely died. Perhaps the fact that I soaked it in TB cleaner helped speed up the process.

Or a vacuum leak.

Doublevision

The fact it does it when warm suggests it could be electrically related to a sensor such as the IACV.