Have you taken a very close look at the TAC plugs and terminals, depending where its mounted they can give problems.
Have you checked all grounds associated with the TAC wiring harness?
I would not look at the ECM until everything else is checked.
Have the TAC re-flashed if possible but I would be looking at wiring and grounds as you have changed the TB alreaby.
Get a scanner and watch the live data while operating the throttle with engine off. It would also be helpful to look at the freeze frame data to see the conditions when the fault takes place.

Neal