What makes an auto tranny go into N at a stop?

Transmissions have always been kind of a magic box for me, especially automatics.



Basically, when I come to a stop, in gear, it will rhythmically try to "go into gear", stalling if I don't keep a foot on the brake & lightly on the gas, or slip into neutral. Seems like it gets worse the more the motor/tranny warms up. '94 4.0 AT, btw.



I'm just looking for a general direction to start looking for troubleshooting.



Thanks