Thermostat or Clutch fan?

Several things come to mind. First you need to ensure the cooling system is operating properly, (cooling fan working correctly, radiator working efficiently, water pump working efficiently and the head gasket sealing correctly). Are you loosing coolant or does the engine boil? If so have the cooling system pressure tested and repair any leaks, if the head gasket is suspect, have a simple chemical test called a "block test" done. Heater cores normally are controlled by a valve. When the heater controls are set to max heat the two heater hoses going to the firewall should both be hot or there is air trapped in the system ,the valve is not opening fully or the heater core is restricted. Autozone have some popular workshop manuals for free on line once you register (for free) on their website http://bit.ly/autozone_repair_info