Oldmobile Omega Transaxle Prob.(final drive?)

First of all let me start off by saying I thank anyone that can help me with this question...

I have an 84 Oldsmobile Omega and it has a AT 125C Automatic Transaxle.

From what I understand when the car goes over 35mph the engine connects directly to the wheels instead of going throught the tranny.

When I first got the car it would go in fine, and stay in, but it would not come out! I would go to stop and it would stall out from being in gear.

Since then I changed my transmission fluid and filter and that stopped for a while... Now I have another problem..

When it reaches 35mph it tries to go into it and then just comes back out, over and over... Sometimes it sticks and then when I hit a bump or a slight wind comes at me it kicks it out.

I really would like to know what this problem is, and if its anything costly. I have asked many people they cant tell me anything...

thanks.

Common problem. Replace the TCC solenoid in the trans side cover.