Idle problems when stopped with automatic but not in neutral

[The Inflicted]

I've got an annoying problem that I hope someone can help me diagnose.

In a 1992 convertible, when the engine is warm and I come up to stop lights I sometimes get weird fluctuations in the idle, with the RPMs swinging from 100 to 1000 and causing shaking. If I put the transmission into neutral or park, or if the light turns and I start to move again, the problem immediately goes away. No other obvious problems or warning indicators.

Any ideas?
I'm not even sure if this is engine or transmission related.

[AZhitman]

That would be caused by an issue that only presents itself when the engine is under load.

Most likely culprit in these cases is a vacuum leak. Visually check all vacuum lines and their connections. Easy to test for vacuum leaks at the manifold - let us know if you're not sure how.

Secondarily, an EGR valve can be sticking open slightly. With higher mileage, it's worth removing the valve to clean it. Simple.

Have you ever cleaned the MAF? Also easy, and can be done when you check for vacuum leaks.

Clean your throttle body bore and plate really well - also can be done when you clean the MAF.

Clean the idle air control valve as well - they tend to get really funky, especially on an automatic KA.

Let's get this thing idling SMOOOTH!