Overheating - Nissan - Sentra - Heating :: Intermittent Heat / Blowing Cold Air

A few weeks ago my car started overheating and blowing cold air while I was on the highway. I took it to my mechanic and he found that the radiator was leaking. Two weeks later the car was overheating and the heater was blowing cold air when I was on the highway. I had it towed to the same mechanic and the car wouldn't overheat nor would it blow cold air. He drove it all around and on the highway but couldn't get the problems I was having to occur. Now I have intermittent heat. Sometimes I will start the car and it will take 20+ minutes for the heat to blow warm air. Other times the heat works fine for a while and then starts blowing cold air. Usually when it starts blowing cold air I begin to smell a sweet odor, possibly coolant. It hasn't overheated again, but I have stayed off the highway.

I just had the thermostat changed, and the upper radiator hose warms up quickly. When the heater is blowing cold air, one or both of the hoses connecting to the heater core do not warm up. Sometimes it's both hoses, other times the hose going in is hot but the other hose is cool. My mechanic seems to think that the head gasket is leaking air into the system, and that the air is preventing the coolant/water from being properly transported throughout the car. He did a pressure test to see if it was the head gasket when I brought the car in but it wasn't conclusive. Does it sound like a head gasket leak, or could it be a blocked heater core, or a weak water pump? So far I haven't noticed any coolant leaks, but I could be missing something.