Overheating

I have a 2004 Jeep Libetry 3.7 Litre with 214,000 miles. It has never
overheated before. Last week, while in stop and go traffic on the freeway, my
water pump broke. I got off at the next off ramp, which took about 10
minutes. It didn't get into the red zone and didn't overboil. I had a mechanic
replace the pump and thermostat. On my way home it started to overheat
and get close to the red zone. By turning my heater on full blast I was able to
keep it in the normal zone. On my way back to the mechanic the next day, it
ran hot again. I kept the heater on full blast so that it stayed in the normal
zone. He changed by radiator but I still had the same problem. He suggested
changing the radiator fan, which I'll do but I don't think it will solve the
problem because it seems to be kicking in when it's supposed to. There is no
oil in the coolant reservoir tank or milkiness in the oil but I am concerned it's
the head gasket. Any thoughts? Am I missing something obvious?