Transmission spontaneously shifts to second gear while highway driving

Then will not leave that gear unless car is put into Neutral or Reverse.
Once back in D or any other forward gear, it will immediately shift back to second.

Turning car off sometimes corrects issue temporarily.
Disconnecting battery sometimes corrects issue temporarily.

For a time, car would operate in 3rd gear [D3] without issue, but still struggled to shift from 2nd to 3rd.
Once shifted to D4, the car would shift to 2nd gear instead of 4th gear.

Car sometimes prematurely and aggressively downshifts when letting off throttle.

-D4 light is not flashing.
-Transmission fluid at correct level and clean.
-Transmission and Locking solenoids changed with no improvement.
-TCU removed and inspected: no apparent shorts or blowouts.