2000 Mustang GT with a 5 speed transmission. While driving, the car starts idling at 2500 rpms for no reason. Sometimes I turn the engine off and start it again and the problem goes away. Sometimes I ignore it and keep driving and it goes away. I have replaced some cracked vacuum hoses and the PCV valve and reduced the frequency of the problem. No check engine light ever comes on and I can’t troubleshoot the problem because it goes away before I can get home and check it out.
Likely a binding throttle cable or a dirty throttle body or a bad idle air control valve. The the list is free to check, cheap to clean and inexpensive to replace in that order. Very likely that none of them will set a code.
Post back with what you find. If it isn’t one of the 3 (most likely the IAC valve), I’d be surprised.