low vacuum problem?

[PeteF]

looking for comments on sudden low idle vacuum. 351W rebuilt by Jasper Engines in 2001, now with 41,000 miles. hyd flat tappet cam giving 365 hp dynoed when new. Holley 750, vac secondaries. I am second owner and engine has always pulled 13 in-Hg at idle over the three years I've owned it. Seems low to me, but I thought it was due to the cam, slight lope at idle. Last few months I notice the engine doesn't feel as strong, doesn't scare me at full throttle like it used to. Vacuum now reading a steady 9 in-Hg at idle. Google advice says causes could be 1) timing (checks good, 10 deg initial, 34 total) 2) late valve timing (can a double roller chain jump a tooth? hard to believe) 3) low compression (checked it today, about 130 on all cylinders cold) 4) intake manifold leak ( squirted fluid all around intake, no change, then removed carb, sprayed gaskets with Permatex red silicone spray, reassembled, checked all hoses, PCV, etc. Still 9 in-Hg after all that. Car runs good, but not scary good, and why the drop from 13 to 9 ! What am I missing......do any of you run that low a vacuum ? thanks for any comments