Sounds like it just may be the nature of the beast :).
Only thing I can add is, make sure if you're setting the timing in neutral.....guessing around 1000/1100rpm's, that you're not into the "curve" at that point. If so, when the rpm's drop in gear, the initial will drop also and cause a lower vacuum reading at that point. I know we're spitting hair's here, but if that's the case, you may pick up a little vacuum by delaying the mechanical curve to a little higher rpm using the 20* initial?
Regardless, run a 3.5" power valve, and enjoy the power that the cam can provide. :D
