Didn't you say that system vacuum measured between pump and N75 was 20" Hg at idle? Shouldn't it be higher?
I had a lot of symptoms like yours on my 335d. It turned out to be a worn O-ring pair on the vacuum pump. It was pulling in dirty air whenever I'd get on throttle. The diagnostic trick I used to confirm that theory was:
- I noticed dirt tracks leading to the bottom of the vacuum pump and wondered what/why.
- I sprayed hairspray around that area.
- Went for a test drive and immediately felt a difference.
So my half-baked contribution is:
- inspect vacuum pump, reservoir, and all vacuum connections closely for dirt tracking that seems to congregate soil near a union/seam/joint, etc
- test vacuum on throttle by installing a remote gauge temporarily
- try the hairspray trick