The promlem is the torque converter clutch solenoid. Gm cars had problems with these and my girlfriends car (91 Buick Century) does the same thing. If you can find the wiring diagram somewhere I recommend disconnecting the connector or cutting a wire to the TCC. The negative side to doing that is a small loss of fuel economy at speeds of over 45 mph. Positive is that its a pain to change the solenoid and it costs money. Good luck.