thermostat

Hey. I have a problem with my thermostat. It only works when i am parked on a hill. when i am just driven around it never leaves the peg it rests on. When I park and leave my car idling on a Hill....the thermostat works and the fans will kick on. I have heat all the time though as soon as the engine warms up. Any ideas about this? I'm just goin to replace the stat but I am curious of why it only works when im parked on a hill?!?!? Thanx.