Am I crazy or what???

O.K. Been working on my 65' cuda for--ever. On the road, runs great, fixing the last little things, and now I've got an idle surge I can't figure out how to fix. 300-400 rpm surge at running temp, not rhythmic, vacuum fluxes from 12-15 inhg. Otherwise the engine runs great on the road. Been thru the carb, checked the vacuum advance, timing, checked the vacuum lines (all three of them). I suspect it's a hidden leak may be in the intake manifold gaskets or something like that. Or could it be the pcv valve? Whatever it is, it's driving me nuts. Any ideas?