I have a 3.0 5-speed that drives just fine, but the engine revs up and down when sitting in neutral, especially after a cold start. The cycles are about a second long or shorter and go away if the revs are held above two grand. I guessing the rev change is almost 500 rpm from low to high. These cycles occur even with the air bypass valve completely removed and replaced with a block-off plate.

Can a manifold gasket leak cause this? My thinking is low intake pressure caused by higher rev without opening the throttle wider* will open a leak which then leans the mixture, causing the engine to slow, which causes the pressure to rise (less vacuum), closing the leak, engine gets better mix, revs up, cycle repeats.
*(Keep in mind this cycling does not require the throttle to be completely closed.)

The vehicle accelerates and cruises nicely, but cold starting isn't quick.

 

I talked to a retired mechanic and he thought, sight unseen, that the PIP might be to blame. I wonder how it would work fine under load.

I did more testing after I left the ignition on overnight and drained the battery. (I wanted to see what would happen if I reset the computer, but not that way!) The rpm swing is larger and slower when cold, gradually getting "tighter" as the engine warms.

 

Try unplugging the idle air controller motor. It may be sooted up and sticky or sluggish and ends up swinging too far either way to try and hit the right idle rpm.

The engine should almost or completely stall when you unplug it and it’s working correctly. If it doesn’t stall or have a REALLY low rpm, try cleaning it and see what happens. If that doesn’t help, you should start looking for vacuum leaks as another issue.

 

It was a loose PCV hose. The hose was still tight and pliable at the valve, but was rock hard and flopping around loose at the front connection to the throttle body. This is hidden underneath so you have to feel for it. Another contributing factor could have been the MAP sensor. I had to replace it after replacing the PCV hose. I'm cheap so I used a junkyard MAP and kept the old PCV and it's running real smooth now.