Could be the thermostat, but just to make sure it isn't something simple; Are you new to volvo's?? There is a fresh air mixture feature that will vent in cold air to the drivers center vent. When travelling fast it will seem to blow more cold air than in town. You can open and close it to see if thats the culprit. Its the far left dial next the center vents. That wouldn't really explain the temp guage, unless you are talking very minute changes.