Strange idle issue - help?

Hey everybody, got a bit of an issue and would appreciate some advice. I've got an 89 4.0 with an automatic transmission, and am having some strange idle issues that I haven't been able to find a solution to online. Jeep starts fine in the mornings, but I've noticed that when I stop at a stoplight, it acts as if I was still giving it throttle, no matter what it is shifted into. This isn't an every time problem, I'd say it comes an goes, with about half of my drives having it happen. Shutting the jeep off and restarting normally fixes it, but recently it caused a huge rpm spike while in park, and scared the heck out of me. My buddy and I thought that the throttle cable/linkage was causing it, checked it out one day when it was doing it with no luck. No vacuum leaks that I can find and I am stumped. Any help is appreciated!

Have you checked your grounds? There's a lot of others who can elaborate on this, but I think there's a few points in the engine harness which could require attention, potentially affecting your throttle position sensor signal.

Otherwise, I'd get some carb cleaner and spray around the manifold. A change in idle indicates a vacuum leak.

Test the ground wire from the flat 3 pin connector on the TPS. Should be less than 1 ohm to the negative post of the battery. Also check it while wiggling wires everywhere.

Second possible suspect is the air inlet temperature sensor in the intake manifold. May be dirty and needs cleaning. Test it with an ohm meter at various temps also against the factory specs.

Look up my old thread here, "The RenX files" for a course on high idle testing and problem solving.

Thanks! I'll be checking the TPS and intake manifold when I get off work tonight. The RenX files looks awesome from my quick scroll through it, thanks for pointing that out to me!