That sounds like to me one of two things. Either the clutches are slipping because of ware or the governor gear is striped. Sometimes too there is trash buildup in the valve body tool. As a passing thought also, the vacuum modulator valve could have failed or the tube clogged too. I can’t remember if that year model had an electrical relay or not either. If there is anything like that you might want to check it out and sometimes the linkage can be slightly out of alignment too. It’s been a long time ago when I use to rebuild transmissions for GM But there is also a heavy spring on the passenger side under a pressure cap. If it broke or weakened it could also cause that same problem.

I hope there is something here of use for you.

Regards

Tony