Heater Quits

If it has plenty of coolant, then a low fluid level condition shouldn't happen......the 'fluid' is the coolant. Be sure the coolant recovery tank is filled to the Max indicator when the engine is cool. Keeping the coolant recovery tank properly filled should purge any air in the cooling system. If that isn't the problem then it's probably in the controls. There may be a water heater valve malfunctioning or the temp blend door malfunctioning.

Hope this is of some use to the problem.....

Dave S(Texas)

Could also be a bad thermostat or radiator fan