Causes for heat to blow hot then cools down, turn off wait 10 min and then hot

Make sure there is no air in the cooling system.

use a laser temperature gun and check the temp of the heater hoses going in and out of the heater core, the lower the flow of coolant the colder the return hose will be, if the return hose is hot then you may have a HVAC ducting air flow problem, if the inlet hose if not very hot then you have a coolant flow problem (such as scaled/restricted heater core, air bound/cavitation , water pump impeller, ETC...)