Transmission going into fail safe.....

[Saintbreezy9]

Hi, I'm new and need some help. About 6 months ago I had to replace my transmission. I took it to a mechanic who gave me a deal and then screwed me over. The transmission he put in the car has a few problems, but I figure it would be better to pin point these problems rather than just start changing out crap. My transmission system will reset every time I start my car it will shift all the way to 4th if I have enough straight road ahead and if I'm doing it manually(my transmission is automatic but can be used as a manual). However, after a short period of time my transmission jumps and ends up stuck in third with the neutral light flashing. The mechanic said that it was my transmission computer so I waited till i could afford a computer and then took it to the closest mitsubishi dealer (150 miles away) to reprogram my computer. After arriving at the mitsubishi place they proceeded to tell me that my computer was fine and my transmission was in fact bad. However, they did not give me any specifics on what might be causing this problem... Anyone have the same problems or know what it might be?????
thanks