Greetings. On very sporatic instances, one of our 3, 2006 xc70s,will violently boil over from reservoir cap. Cap was slightly loose when this occurred. This time, cap was on tight and the heater core component burst. Not sure what the fail was specifically as I haven't gone in yet. Water pump fine. Radiator fan fine. No visible leaks aside for a small wisp at thermostat housing. Never see the temp gauge rise. Thinking blown gasket would show consistently. A stuck thermostat would overheat from cold engine, not an engine already at operating temp. Also, when topping up reservoir from a boilout, it will receive coolant, then take more after thermostat opens. Can drive around for weeks with cap loose and all of a sudden, coolant would gush past the cap and dump a 1/4 gallon. Fill it back up and good for another few weeks until it happens again. I believe a sudden over pressure burst heater loop somewhere. This is not a normal occurence. Wise and thoughtful input would be most appreciated.