How does the torque converter clutch behave when this is happening? You should be able to see it lock up when cruising at >50 mph by watching the tachometer. It will drop about 300 rpm when the TCC lock happens. I think the normal engine speed is ~2000 rpm at 60 mph (maybe 58 mph) with the transmission in OD & TCC locked up.
Are you getting any blinking of the O/D light? If so, have you read the codes to find out the reason?
Also, did you check the level (at operating temperature, on a level surface)?
Transmissions need tight seals in the hydraulic passages. They can behave strangely if they lose pressure due to any leaks. I don't know for sure it was the cause, but before my recent problem with the main seal falling out, I'd lose TCC lock-up on long trips after driving 100-150 miles. Since that seal is near the torque converter I wonder if the two were related. I haven't seen the problem with the TCC since that was repaired.
I should probably add the PCM 'decides' when to shift gears using data from the engine and transmission sensors. It calculates the 'engine load' (in percent) and if that number is too high it shifts to a lower gear or if it is too low it shifts to a higher gear. It is possible that a faulty sensor is leading the PCM to change the gears incorrectly.