1995 Buick Regal 1995 Buick Regal wont shift into overdrive

This problem is very strange. The car will shift normal, perfectly fine. Eventually it will decide it wants to go into 3rd and stay (it wont go back into overdrive). Now the really fishy thing about it is that I have to turn my car off, get back up to speed, and the car will shift back into 4th. What is even worse is I cant slow down to 0 and then get back into 4th I have to turn it off!

Please help, I have no idea what is wrong and a mechanic said he cant help me. No recommendations or anything.