The tranny has been rebuilt twice, but im having problems with the car slipping out of overdrive. Its a 91 Trans Am 350 TPI, all the gears shift fine until u get into overdrive, or last gear it shifts. usually when u hit 40 it shifts down to 1200 rpm, but now it doesnt want to, and once it does, it will stay there, but once u give it just barely any gas at all it will slip out out so easily. I checked my fluid, appears to be fine. Could it be my modulator? Or my shift cable?

Could it possibley be your lockup that's slipping in and out? I had a similiar problem and that's what it was for me. An easy way to check that is to take off the transmission pan and take out the lockup selinoid, it's just a platic thing with a couple of wires running to it held in by 2 10mm bolts or something like that. Its has a check ball in it. If you take out that check ball and put it back together, it eliminates your lockup. So if the problem goes away at that point you know it's the lock up slippin in and out for some reason. In my case it was a bad torque converter, but there are other reasons the lock up could slip in and out like that.

ill definitely look into that, it makes sense, it could be my problem.