Shetara

I have a 2004 Subaru Outback Wagon. 180,000 miles.Head gaskets were replaced about 3 years ago. Due to overheating issues, just had a new radiator and themostat put in. It ran fine for a couple weeks, just in town driving. After driving up to the mountain, it stated overheating again. No signs of head gasket issues (no whit smoke, no smell, nothing leaking, no milky film on the oil cap)...my mechanic said there was probably air in the cooling system. Possibly coming from the head gaskets? Did some research and burped it a few days ago. Ran ok for a couple days then started to over heat. (Maybe I didn't burp it long enough?) Here's the catch. Before (and since I burped it) I've been driving around with the heat on (no fan) with a meat thermometer in the vent. When it was driving fine, the meat thermometer would read 160-170° and the car's thermometer would be mid way- where it mormally is. Now when the car's thermometer says its overheating, the thermometer in the vent shows normal temperature. What gives?! I am currently re-burping and air is coming out. I read that burping could take 2 hours. Could it take more?