Intermittent heat

I have been having a problem with my heat lately. Maybe someone can help me out. My heat comes and goes. I changed the thermostat, but am still having the same problem. I haven't picked up any pattern yet, it just seems to come and go as it pleases... and I am in Michigan, so I have had some very cold drives lately. Any advice would be greatly appreciated. Thanks!

air lock maybe from either the thermostat replacent job or a compression leak eg head gasket(s) , try replacing your rad cap, and feel the hoses goin in and out of the heater core