This thing looks like to me as a bad tcm.
Transmission control module or the contacts or grip requires cleaning. Other than this see for the throttle sensor voltage it should be 0.35-0.45 on idle. If the readings deviate too much it messes with the shifting pattern of the car.
There must be some code. Basically it requires a good scanner like, launch, gscan 2,Carman lite to pull out the tcm codes. Since many obd2 diagnostics show only engine codes only and this matter is linked with tcm codes. So find a guy with the good scan tool. And try finding tp voltage. And see for the tcm. Remove its grip and check for the the good contact, there shouldn't be rust or water seepage there.