Transmission error code P0772

History: The TCM had been replaced at 125,000. It seems to be working fine. Transmission fluid had not been changed for about 70,000. I ask my son to bring it back to change the fluid around 50,000 but he was busy with the school. Until a month ago, the AT like came on. So I flushed the transmission fluid and notice that it so thick black and a little low. I changed the filter and fluid. Cleaned the oil pan. And it was okay for about 2,000 miles. The jerking and clunk sound appeared when the shift moved from park to reverse. The drive shift seemed to be okay. No slipping issue. Drove on the fwy for forty miles no issue. I notice that the fluid get dirty very quickly. But I cannot smell any burn. Yesterday the lights came on both engine and AT. The error code read P0772. I suspect that the dirty fluid is causing the problem. I put in Sea foam - Hydra transmission tune about 12oz last night and drove for a few miles and this morning. The shift is smooth as normal.

Questions: the part that the fluid getting dirty very quickly that is worrying me quite a lot. I plan to flush the fluid and change the filter one more time this weekend.
Any suggestions what could cause the fluid getting dirty quickly. Could it be a mechanical failure? What should I check or look for when I change the filter again this weekend. Should I change the speed sensors and shifting solenoid?
Any help is appreciated!
Kris