Transmission slips only at operating temperature

4T65E transmission is cold=all gears lock in, and shifts correctly.
Easily spins wheels on sand roads.

At operating temperature and trans full=all gears slip badly.

Turn off engine, sit for five minutes, then restart car=transmission works okay for very short distance, then slips badly again.
After restart, I drive down steep hill to fifty to sixty mph and am able to maintain speed for several miles, even over medium hills if I do not stop or slow much.

Changed fluid and filter=problem remains.
Poorly maintained, fluid was dark brown/black, but did not taste or smell burnt.
Silt/dust in pan, but no chunks of anything nor aluminum shavings.

Replaced TCC solenoid=problem remains.

Have a code-P0757(shift solenoid B) but do not understand how that is effecting all gears, and only when warm.

Car shows over 211,200 miles, and do not know if transmission is original.

Thank you for your advice.