Heater only blows heat about half the time

Heater only blows heat about half the time. I've replaced the thermostat, heater control valve, the t by the heater control valve, radiator cap, can't see any visible leaks. It will often overheat for a second then you hear a gurgling sound and the temp drops way down. When I give it gas I can alsohear the gurgling, its coming from passenger side around the heater core/hoses/ reservoir tank. Any suggestions other then to take to a mechanic?