1992 Chrysler New Yorker Intermittent transmission problems

92 Chrys. New Yorker, 3.3L V6, 4 sp. Auto w/ OD

I bought this car from my girlfriend and when she bought it, the transmission's 4th gear didn't work at all. She had a used transmission swapped in as well as a used transmission computer since the old one was deemed faulty. All has been fine up until now.

I've been driving the car for about 6 months, and just suddenly the transmission started acting up again. Everything works fine until being driven a while and getting the engine nice and warm. Then at random intervals when I'm slowing down from 55mph to corner or come to a stop, once I get down to about 35mph, the transmission will drop into 1st gear (quite hard) and stay there.

Shifting the car to neutral and back will not fix the problem, but shifting the car into neutral, shutting the engine off and restarting it, then shifting back down into Drive will. The transmission will work properly again after that at least until it decides to mess up again.

I checked the fluid level and it's fine -- I also get no lights on the dash when it happens. I'm pulling my hair out. I've taken it to my mechanic, but since it only acts up at random (about once a day now) he says he can't diagnose it. Help!