90 GP Transmission Problem. No 3rd or 4th!

I have a posessed 90 GP Le coupe. Everytime I fix one thing something else goes wrong. This is a 3.1 car with the 4 speed automatic 4T60M I THINK. Can't recall for certain off the top of my head. Anyway, from drive it goes through 1st and 2nd fine but the shift to 3rd is the same as going into neutral. Obviously something is wrong, but am wondering if anyone can help pinpoint the problem. I'm thinking either the pump, the 3rd/4th clutch packs, or the torque converter. The fluid level is fine and all vaccuum and electrical connections are in place. I'm resigned to the prospect of tearing it out, but would greatly prefer to have a good idea where to focus rather than going exploring. I have a GM manual for the car and all their in-car tests check out, so it has to be internal. Any suggestions are appreciated. Right now it's sitting under snow in the driveway and will stay there till it warms up.

There are two possibilities here, electrical issued caused by a bad computer or solenoids that control the 3-4 shift or a bad transmission altogether. You can have this diagnosed by a transmission shop and this would be the best way to have it checked. If the solenoids need to be replaced this is about a $500 job. Otherwise you will need a new tranny or a rebuild.