First you need to check for malfunction codes as i said a bad adjusted TPS will cause this also a bad adjusted throttle cable ,transmission fluid level to low .
All these check out good then it starts to look bad like a bad overdrive solenoid or clutch disc.
these are the typical issue ,
then comes the overdrive piston failure Valve body malfunction .
If the fluid is still clean and nor gone dark brown to black then it may be just a valve body issue ..
Have a good transmission shop check / test the transmission a bad fluid pressure could also cause you
this issue so pressure test on all test ports are to be tested .
If you can't do the testing yourself i suggest you find a good transmission shop that has a good reputation
to test this for you .
Still need help then please get back to me with the reply button, and not the poor or bad service, this way i can get back to you or suggest another expert if needed.
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Kind regards Greg.