i have a 4 year old house, with oil heat- baseboard hot water. the house is a bi-level but we only use the upstairs for now. there is one thermostat, upstairs, centrally located. we've never had any problems heating the house comfortably until now. the thermostat does not seem to be properly regulating the temperature of our home. it's either too hot or too cold. if i set it at 67 it will heat to 70 or 72. could the thermostat be broken? how can i check it or fix it? i just recently had the oil burner serviced for the season, by a professional. could he have changed a setting or something that affects the thermostat upstairs?

thankyou much

The thermostat has an adjustment for how many cycles per hour it will run. A more common problem is the placement of the thermostat. I will assume the upstairs is comfortable and the lower level is too hot?
The baseboards may have a small ammount of air in them, causing them to heat the upstairs slowly. this also causes the lower level to overheat. Check the baseboards for air on both levels( air bleeder valve on either end of piping) then see how the system operates.