Check the coolant level in the radiator, not in the reservoir tank. If you have an air bubble in the radiator, the heater core has air in it and you'll get erratic heat/coolant flow in it.
If the radiator is full, when the car is fulled warmed up meaning the lower radiator hose has warm water circulating, run the motor at 2500 for 10 seconds and then repeat that 4 more times. That shoud get the air out of the heater according to the service manual. Then bleed the air at the thermostat housing using the little screw on top.