This problem should be a relatively straight forward fix, especially since you threw a bunch of "maybe" parts at it. Not the best approach, but will make it easier to diagnose now. Do you have a VCDS where you can monitor what the TCM is commanding the trans to do? Knowing whether it is commanding a forced downshift will save us a lot of diagnosing time. There are some rough and tumble ways in determining this with the ignition on, engine off, but the VCDS would be preferred.