95 Cutlass tranny/overdrive shift problem

1995 Cutlass Supreme with 4 spd auto tranny w/overdrive. Problem is that after driving for about 15 miles and the tranny has time to warm up it will shift out of overdrive back into 3rd gear and then stay there till you stop the car, shut it off and restart it. Then it will go back to highway speed, slip into od and then back to 3rd and stay there. Considering it resets itself when I stop and restart it could this be an electrical problem, torque converter or is this tranny about to crap out?

any suggestions or similar experiences would help.
:banghead:

1995 Olds Cutlass Supreme Convertible
2002 GMC Sierra Z71 SLT

Sounds like your modulator that controls converter lockup is losing vacuum. pull the vacuum line off of the modulator (right below your airbox), if any tranny fluid comes out, your modulator is bad.

This is the same problem I have had for a few years.

Did this fix the problem ?