Just realized I never shared the solution on the Pontiac cooling system issue…
It was a worn/cracked hose in the rear of engine, hard to reach, difficult to see. It was so deteriorated it almost crumbled in my hand, but it was mostly worn on top (closest to the engine heat).
That, plus the old radiator cap, was probably enough to suck the air into the system. But a bit ticked at the mechanics who missed this, which really should have been noticed during the radiator flush.
Nonetheless, thanks to everyone who actually had some constructive advice instead of just saying head gasket. (If you haven’t noticed, it’s more than a “pet peeve” when people blame the head gasket as an excuse to give up on a problem)