You might try a 12.5 power valve if you can find one. I have about the same vacuum as you do and I'm running a 10.5 power valve like you and am having the same issue.
As soon as I get it back together I'm going to try a 12.5 power valve.
IMO, if you are running close to being on the main jet at a cruise you want as little time between cruise vacuum and power valve opening as you can get. That's my guess right now, but I haven t been able to test my theory yet because the brown truck hasn't brought my scatter shield back yet.
