This is in a 1999 GMC Sierra Regular Cab with a full bolton 5.3.

The transmission is a Rock On Built transmission with a Yank TT3000 converter, Transgo vacuum valve body, servos, etc.

For some reason when I command TCC lock up below 39 mph in 4th gear, the lock up gets harsh and starts darkening the transmission fluid shortly after eventually requiring a fluid change or transmission issue increases.

Change back to 39mph lock up, no problem at all, and no issues continue.

What could possibly be causing this problem?

I am attaching my tune to see if there is some problem with it.

Attachment 14828