FWIW my moneys on a bad head gasket. I don't see how bubbles get into the radiator if it were a bad intake gasket. I did however have a bad intake gasket on a 4.3L that caused coolant level to drop. I'm thinking that alls ok(except bubbles in rad) until the engine gets hot. Then the coolant gets pressurized and is pushed into the chamber causing odd combustion/pull on next intake charge. Vacuum gauge should be fluctuating. Just a guess?
Any chance there wasn't enough antifreeze in the coolant mix so that a small amount in gasket pocket could have froze/expanded gasket enough to cause leak?
I'd look for an odd colored plug/plugs and perform a leak-down test or just hook up some compressed air(120psi) into spark plug hole(piston at TDC on firing stroke so that valves are closed) and listen for noise in intake/crankcase or bubbles/pressure in radiator.
Hope its not, but you should check if you can't find a vacuum leak on intake somewhere IMO.
Hope some of this helps one way or the other.