Mitsubishi Repair: Automatic Transmission trouble., automatic transmission trouble, mitsubishi 3000gt

Question
I have a 1994 Mitsubishi 3000GT,3.0 DOHC,non-turbo, automatic.  While on overdrive the transmission automatically changes gear to third/second.  Not on first/forth.  Especially forth. I would turn off the car and turn it back on, and it would run.  But then again it happens.  Would you tell me what is the cause of this?  
Thank you

Answer
Tom,
This may be caused by a faulty transaxle control unit (TCU).  This is the computer that controls the selection of the gears relative to the speed of the engine and speedometer.  I would recommend you have a transmission shop inspect the transmission and the TCU to see if they can detect any problems.
Good luck!