Not sure why a "vacuum leak" would be intermittent.... regardless of how you ran the engine -- seems to me you'd either have a leak or you wouldn't.
If you can pull the throttle blades back to get to your target idle.... regardless of how you drove it... then to me it's saying you're hanging up on the throttle blades AND OR -- have you checked your float settings etc? Could you be a little on the high side and once you have some heat in the engine/carb you're pushing fuel in. BUT if you can, as stated above, always return to the idle you want by moving ("pushing on") your throttle linkage then it's a linkage/throttle blade issue not a fuel or vacuum issue. Since you have a dual spring return it should certainly be closing the linkage. So take a look at the STOP when this happens before you touch anything and see if there's a gap between the stop and your linkage. If there's no gap -- then you have a fueling issue that you'll need to solve. A combination of relatively high idle speed (900) where you may just be on the cusp of uncovering the transition slots and just a barely sticking issue could be you're problem. But I think you would feel a sticking blade, especially if you can reset with your hand.
I would NOT recommend holes in the primaries. That is an issue/fix for wild ass cams and high idle speeds which is a cover up solution when you really MIGHT (since we don't know what carb you have) need a different carb with changeable air bleeds etc. Not being rude to the poster that mentioned this - but holes in the primaries is an old skool backyard remedy. Yeah it can work - but you're killing the carb for any other use on a different combo it. Yeah it can be repaired - but it is still just a cover for a proper fix IMHO.
Your float check should be done when the engine is HOT.... because todays fuels expand more than old stuff did.
Also -- Holley had a serious problem with some bad metal for awhile. The new gas formulas where eating the metering blocks etc. so have you looked at that? Have you had the bowls off the carb for inspection?
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