I just want to say both these guys are on the money as far as possibilitys.i just finished a similar problem myself.was a manifold leak.very small at that.i run about 10" of vac at idle and would fluctuate around 3 " slowly.now with that fixed it runs at right around 10" at 1000 rpms with a +-1 " due tp the extreme cam.just a reminder dont miss the little things cause they will haubt you till the end then youll kick yourself lol.good luck