service the auto. USE DEX 111 and AC Delco filter kit with a neoprene gasket, all up $50.

The Type F trans fluids (FORDS etc) shift differently to the GM trans which commonly use type D fluids like DEX 111.

Drag racing guys use type F fluids in GM trans because the shift is quicker than the recommended type D fluids..

or your TCC solenoid may be on the way out