TCC Slippage.
how the pressure gets to the TCC, starting with the force motor
(PCS). TCC gets a duty-cycle fraction of that. At cruise on my
car, stock, the PCM was commanding 0%, minimum line. That
was not enough for a 10" TCI and maybe not enough for yours
either.
Get an idea of the Delivered Torque values where you slip and
the commanded force motor mA or duty, whichever your tuning
tool may show for table axis. You need to give it more (and may
as well set TCC Duty min, max to full-on and try to kill any "slip
learning").

