I have a 94 bonneville that is driving me crazy. Driving on interstate running fine, all of a sudden, comes out of overdrive.You can cut car off, and restart, will work a while, then do the same. Recently it has started something new. Car will start to go up and down in rpms, not stay steady, then straighten out.I have replaced the transmission, thinking that I had a problem inside. Same thing with new one. Also replaced the speed sensor on the transmission, and throttle position sensor. HELP. This is the most frustrating thing I have ever run into. Any suggestions would be greatly appreciated. Thanks in advance, Steve

Do you have a service engine light? PUll any codes. https://www.gmforum.com/specifications-general-information-132/obd1-codes-retrieval-gm-1983-95-cars-trucks-%2A%2A-278820/

have you checked fuel pressure? Inspect your plugs, take a spray bottle and mist the plug wires and coils with water while the engine is running. Do this at night and turn off the lights. Do you see any blue flashing?

Ok check your torque lockup control, its what keeps you in fourth gear, and mine does the chugging issue i think it is the mass air flow.