90 Lumina 3.1 AT Issues

My girlfriend's 1990 Lumina will not upshift unless I remove my foot from the gas pedal and it happens for sure from 1st - 3rd gears as well as hesitating on acceleration. The trans fluid level is okay and I checked the vaccum modulator and it was bad. I replaced it and the car still exhibits the same sypmptoms. Also, when I floor it, the engine revs up but instead of downshifting, it acts like it is in neutral (not in gear). I take my foot off the gas and it goes back into gear. The TV cable was adjusted and still the same. The VSS seems to be okay as the speedometer isn't presenting any problems. I'm running out of things to check on this car. Please help me.

My 1990 Lumina had the same problems I was told by a shop it was the transmission interlock and this is what is causing the problem. I think that was the what they said. If I took my foot out the accelerator it would buck and hesitate and lose power unless I down shifted or accelerated. Its not something easily serviceable. Its in the transmission.

After reading several posts on this forum, I think it may be the throttle position sensor. I'll post back once I change it and let everyone know how it goes.