Yes, based on my vague recollection of such problems with a B20F (similar to your B21F), the AAV is a good suspect. Does yours have the adjusting nut? When they start to get a bit weak in response, you may be able to try tweaking them by forcefully rotating the bi-metallic sleeve a bit to respond a bit more sensitively. As I recall, it seemed to help, but then I was probably trying all kinds of other things. I only recall replacing the AAV once with a new one, then mostly out of desperation, not knowing for sure if it was the entire source of the problem. Keeping the entire intake side clean and tight, making sure there are no injector seal leaks, is just as important. Do re-check your injector seals when the engine is stone cold by spritzing around the base. The seal will improve as the engine warms so it can be deceiving. Good luck.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now