A750F transmission problem
it might be in limphome mode.?
why not scan it , and see what DTC errors there are, from TCM.
or do all the tests in the FSM for , ON CAR SERVICE.
FSM= factory service manual.
ever try a tranny shop>?
The key to service is doing the steps in the correct order
1: tranny ground trap good. (missing or damaged)
2: scan it.
3: check pump pressure and do the stall tests stated in the fSM.
4: level checks, and if wrong ATF in tranny ?
5: inputs to TCM wrong, ???? ECT, TPS, VSS1,2,3
6: reset TCM and try to see if learn mode , corrects bad shifting.
OTTOMH, there are more. lots more, A/T are complex.
lots to fail.