Ok so technically it's not a Volvo, but what I learned should apply to all cars.
I noticed that the temp gauge was up to around 2/3 coming back on a 2 hour drive about a week ago - the temp gauge NEVER goes over halfway on this car (even when you lock your keys in it with the engine running and AC on when it's 108F outside).
Moral of the story - when your car gives you an indication that something is wrong, DON"T IGNORE IT.
Thermostat failed completely today in the stuck closed position. Took me 2.5 hours to get it back home 10 miles or less. Drive for 2 min, stop for 20. Repeat.
Pics of the carnage: (yeah I managed to shear off BOTH bolts on the thermostat housing)
Here we go - sorry my camera sucks at close ups they never come out in focus.
Calling it quits for tonight. Vicegrips didn't budge it, and I'm not going to FUBAR the threads. Time to buy a socket stud extractor.