Check the temp rating of the installed thermostat. It is stamped in the copper temp sensing bulb of the thermostat. Usually in degrees Celsius(82, for example) My car ran hot for a while and while throwing parts at it, I found the new thermostat to be too hot. There are two temps available frome Volvo. I would suggest the lower one for your climate. I don't remember offhand what the temps were.

Also, check the radiator cap, or in this case, the pressure cap of the resevoir. If it doesn't hold the proper pressure, the water will boil in the engine block. Check manuals to find the proper pressure rating for your car. Volvo makes caps with a couple different pressure ratings, and I believe they are color coded(black, gray etc).

Make sure you have a 50/50 solution of clean antifreeze and water in the system. Running staight water is not a good idea. Antifreeze provides rust and corrosion protection as well as raising the boiling point.

Finally, a cooling system pressure test and a compression check may be in order to check for a blown head gasket.