2000 Honda Accord low heat

No heat diagnosed as a plugged heater core. It was flushed and there was heat for a few weeks. Again no heat. The engine and Core were flushed again and there was heat for a few more weeks. Heat went away and we were told we need a new heater core. I replaced the T STAT and heater control valve with Honda parts and had heat for another week. Over the summer the car would occasionally run hot. This happened 2 or 3 times, never overheated though. AC always worked fine. Low/no heat again this winter. I found the overflow reservoir almost empty with no visible leaks. I refilled it and there was heat again for a few weeks. The Blend door sound like its moving ok and you can feel the change in airflow so I think thats OK. The heat control cable is definitly moving the valve OK. Is this a possible head gasket issue cauing low coolant and the system getting airbound? The Lower (inlet) heater hose is steaming hot and upper is also steaming hot with the heat control on hot but no fan. With the fan on the lower hose stays steaming hot and the upper hose stays hot but gets cooler which would tell me that there is good heat transfer in the coil correct? There is relatively low airflow in the heat position. Whats the chances of really dirty cabin air filter giving me problems?