350sd vacuum

[sarnot]

Long-time-lurker, first time poster
I have a 1990 350sd with some intermittent problems: erratic shifting, erratic climate control. I figured it might be a vacuum issue so I bought a MightyVac, checked for leaks, found a bad connector and replaced it. Now when I check for leaks I can hold vacuum for some time...so I figured all would be better but it is not. I noticed in stop and go traffic that the defrost vent would open while driving and close after sitting at a light. So I hooked up the mightyvac and found that at idle I have 23hu" (am I saying that correctly--I have good vacuum, I guess) but when I rev the car up I lose my vacuum. The longer I keep the revs up the lower the vacuum will go, nearly to zero it seems. I'm guessing this is not right? Any suggestions?