My 96 XJ12 is idling high when I start it up but quickly starts to rev up and down a few hundred revs over and over but as soon as I drive away or put any load on the engine it seems fine but put it in park and it revs up and down again. Any ideas?

 

Any Check Engine codes?

It could be a problem with the idle switch, which is part of the throttle position sensor. That might set a code. Tests can be made at the TPS plug. I will have to look up which pins to test for the switch and operation of the TPS. Stand by.

Next I would check the idle air control valves at each side. They are known to get dirty inside. 4 small bolts hold the top part on. I think there is a small coolant line attached too but it won't spill much.

 

There are 2 speed sensors inside the GM transmission that talk with the transmission ECU which in turn talks back and forth to the engine ECU so they can dance together effecting engine regulation kickdown

A fault usually accrues as you bring it out of park into drive and goes into the first gear and low RPMs

This would be a cleaning of the connector on the transmission body

This accrues with the inline 6 and a different transmission so is a stretch if the same accrues in your powertrain combination

You might be able to clean the transmission connector without jacking the car

 

Idle switch is closed at idle, with continuity between IDL and E2 pins. Then it opens shortly after throttle opening. This may or may not be related to your issue but good checks to make anyway. The TPS and the idle speed control valves are OBD monitored so a major test fail should give a code.