UH, that would be bad news, as we just did the head gasket job a couple months back...

Curse Volvo for not putting a high temp light on the dash. My daughter didn't notice the temp gauge had crept up until the engine make a "groaning" noise. I really hope that's not another head gasket.

So, there's nothing else that could cause the reservoir bottle to blow out liquid and gas when it's very cool other than the engine compression you mentioned? I was thinking maybe the thermostat was failing closed, opening only a little at times, then closing off before the system had cooled down, trapping water under pressure in the coolant system (top half).

This does not sound good