Volvo: Tranny slippage?, volvo 740 turbo wagon, volvo 740

Question
Don't know a whole lot about cars; bought a 92 Volvo 740 turbo wagon back in January and have been learning bit by bit as I've had to have work done on it.  Driving across the country about two weeks ago, I had a problem where the RPMs were running really high - seemed like it wouldn't shift into the highest gear.  Not knowing anything about ATs, I stopped and took it into a shop the next day, only to discover on my way there that it was driving normally again, which they in turn confirmed.  Didn't have any more trouble until a few days ago, when the same problem popped up twice.  Both times I pulled over, put it in park for a minute, then tried driving again and all was fine.  Hadn't driven it again until today, and now the RPMs are high (3k at ~50mph) every time I try driving.  When I stopped during my trip to get it looked at initially, I was told that it had probably "slipped" out of whack for some reason and then gone back.  Is that common?  Whatever the case, it now appears to have gone permanently out of whack.  My guesses as to what may have contributed to the problem are that (1) I was told my AT fluid was "burnt and overfull" back in January and, like an idiot, decided I would put off getting it flushed to save money (I was quoted about $200, which I didn't have at the time) and (2) the first time it happened was immediately after I had pulled over and shifted into park while I was still coming to a stop (10-15mph?) because I was doing too many things at once.  Can you tell me whether the tranny is shot, and if so, whether it would be worth it to replace in a car i paid $2800 for in the first place?  Sorry to be longwinded, but I wanted to give you all the info I had.  Many thanks for whatever input you can lend.

Answer
Taylor,
By what you are telling me it sounds like the trans is not shifting into overdrive (4th gear). If it does not shift then it will remain in 3rd gear causing the RPM's to be on the high side. Does the overdrive off arrow on the dash happen to be on when this happens? What is very common and will cost about $60 is an overdrive relay. Located in the rear of the fuse box. Behind the ash tray... $200 for the trans flush is a bit on the high side. I usually see that job costing $125 to $150.
To replace the trans in your car with a used unit would or should run about $1000. Not real bad, if you like the car then it would be worth it.