If it's going up and down there Groundhog, the thermostat is working.
Typically it climbs slowly, then dips quickly. It should not rise to beyond 12 O'Clock on the wee Volvo temp guage (on mine anyway). If you put your hand on the coolant pipe after it leaves the engine, you will feel it heat and cool alternately as the thermostat opens and closes.
How are you judging the overheating?