At this point, it is hard to say exactly what is causing it. Since it doesn't overheat at highway speeds, I feel the water pump and the head gasket should be fine. Understand, the head gasket doesn't care if you are moving or not. I really think we should start simple. First, when it is overheating, look at both the upper and lower radiator hoses and make sure they are not collapsing and restricting coolant flow. I also need to know if the heater is working properly. If not, there could be a simple air block in the cooling system. Next, if the hoses are good and the heater is working, I would recommend flushing the radiator. It could be partially plugged and causing this problem.
Let me know if you can do these things and what you find.
Thursday, July 3rd, 2014 AT 8:03 PM