No heat when sitting still, hot when going

Could be one of a couple of things. May be low on water / coolant. Check the level when it is cold. If it is low and that fixes it you need to check for a leak. The other possibility is a partially clogged heater core. A partially clogged heater core does not allow enough water to move at idle so the air blowing across the heater core cools the water to fast.