[cabmandone]

If you have a voltage meter, put the leads on W and C. You should see 24 volts if the thermostat is calling for heat. If not you'll get no voltage. You want to make sure the call for heat is ending and it's not your furnace cycling off of the limit switch. And easy way to test the thermostat is to put a jumper wire between R & W at the furnace circuit board. If the furnace just keeps running, it's the thermostat. If it cycles off again, it's cycling off limit and you've got some looking to do to figure out why. Typically a dirty air filter but I've seen dirty evaporators cause the problem and a blower wheel that is slipping on the motor shaft too.