Friend that I am helping out 383 has all of a sudden started to run poorly. After checking to make sure the cam isn't going flat he got it back together and I start see whats going on , he adjusted the timing without a light

Anyway when it's idling at about 900 rpm the vac gauge at the base of the carb has the needle fluctuating wildly, 10" or more, bring the RPM up to about 2k and the needle smooths out ?

The engine is fresh and it was running well till the other day it just started running poorly, carb is a fresh rebuilt Holley 650 DP that's about 30 years old.

I'm a little stumped and don't want to tear the thing apart further than necessary, a search online says sticky valves or a vac leak??? I sprayed starting fluid around the carb and intake thinking big vac leak but that turned up nothing ? There are paper gaskets on either side of the valley tin.