My 1990 Cherokee will start up just fine; but it will idle at different revs about every 20 seconds it will go up higher, then drop down. This is only in park.

Any idea what it could be?

 

This is addressed in your other thread. Read it first and then see if you have an answer.

 

no i don't see an answer in there.

 

i didn't see your other thread, but, do you have a/c, and is it on, or at least the defrost setting selected with either cool or heat.

with an a/c equipped jeep, your a/c pump will run whenever you have any a/c modes selected, or your defrost. doesn't matter if it's in cool mode or heat mode.
you will hear the pump kick in and out about every 20 seconds. it will sound like the engine revs up a tiny bit when the pump kicks in. then back to idle when it kicks out.