Tranny problems - help needed!

[1995_riviera]

Hello, I need the forums help again.

My 95 Riviera SC has had some transmission problems in the recent past. After about 15 Minutes of driving, Overdrive and 4th gear are gone and I´m back in 3rd. If I stop and pull the key, it will work be fine for some minutes after restarting...I wanted to sell it in this condition and let the new owner bother about reparing it. But:

Today things got worse: When stopping at a red light (still in ""D"), the car wants to go and only the brakes are holding it back - the clutch doesn´t "open".

This wasn´t enough. While driving, I wanted to accelerate and the usual power just wasn´t there. It took ages to go faster. But there was no slip - rpms didn´t go up. When in neutral, the engine revs just fine.

I think I collected all the things that can go wrong in one car!

Now to my question: What will I have to replace? No one here in Germany knows this car and no one is willing to repair it without further info...

I hope you can tell me more, I´m lost without your help.

Greetings,
Lutz