I have a couple of suggestions. 1) Turn your gas pressure down. You should be at 10 lpm, not 20 lpm. This high pressure may well be causing turbulance. 2) Although it is possible to use thoriated tungsten on aluminum, you will like the results of pure tungsten better. 3) Balance your sine wave or use more positive on it, for better cleaning action. 4) Switch back to all Argon, you will get better cleaning action with that than a mix. 5) Use the stainless brush. 6) Make certain the parts are 100% dry. If you are cleaning them, rinsing, wrapping in towels and then locking them in a plastic bag, I wonder, are they dry when you weld them? Good luck and if you solve your problem, please let us know how you did it.
Respectfully,
Mike Sherman