HELP! Transmission sometimes wont go to 3rd gear

I have an automatic 1993 g20 with 120,000 miles on it. The transmission has been giving me some issues in the past weeks. Sometimes the transmission wont go past 2nd gear. So the engine revs up extremely high. The way I fix it is I stop the car, turn the engine off, then turn it back on, then I move up and down the gear shifter (remember it is an automatic) Then it usually works after that fine. More then half of the time the transmission works great and no problems. There is no pattern to it. I know the transmission is a hard thing to work at and most people don't have knowledge of it but any clues on what the problem is? My mechanic who doesn't work on transmissions says it could be as easy as something loose inside. What would it cost for a rebuilt transmission? Any help on this issue would be wonderful. But I will always love my g20, great car. Costing me a lot to restore it though!

Whats the transmission fluid like ?

i would take it to the main dealer and let them plug in the consult diagnosis machine and see if there is any fault codes stored in the system. this may save you some money in trying to fix it.