Mitsubishi Repair: Transmission problems, pulse generators, transaxle

Question
I've been working on a '92 Diamante with the F4A33 FWD Transaxle. In the begining I think it was in Failsafe mode. But the guy wanted me to change transaxle with another "junkyard" unit. Did that, now it jumps and dies when shifted into reverse. Do you have any idea why? I've changed the fluid and filter and swapped pulse generators.

Answer
James: Can you coded the computer in the car to see what it might bring up as to the problem? The computer controls everything in that car, if it was in failsafe mode the computer would have put it there. Maybe you just need to repset the computer, or maybe the computer is bad. The car is the right year for the bad computers, so guess I would be checking the computer its self. God bless: Linda