Why does my Camry keep eating transmissions.

My 96 Camry's 4 cyl engine siezed and my garage replaced the engine with one from a junkyard also a 4 cyl. When he installed the engine and test drove it all was fine. He pulled it into the garage to check something over and when he went to back it out the car did not have reverse.

He decided the transmission also had went although it was working fine when the car went into the shop. After installing a junkyard transmission he test drove it with the same result.

Deciding that he just had bad luck he exchanged the transmission for yet another. Got it up and running and this transmission was also bad. So one more time a 3rd transmission from the junk yard went into the car. The car worked fine for about 30 miles when this transmission also gave out.

The mechanic decided to refund all my money on the transmission and referred me to a transmission shop to make the repair.

The transmission shop assured me they would take care of the car and totally rebuilt the transmission and reinstalled it. I picked up the car and it ran like a top - for about 40 miles. The shop had the car towed in and decided it needed a new valve body (?). When they opened the transmission to replace this part they found so much shaved metal they called me back and are taking the whole thing apart again.

All the garages are reptuable and really doing there best to get this car going but I am thinking they are missing something simple - like perhaps they are putting in the wrong transmission or something. Or there is something wrong with the junk yard engine that is causing all these problems.

Any ideas?