Overheat with strange assumptions

Hi there,

I gave this car to my brother to use since he needed a ride and was working on Dallas. After about eight months of usage, the car began to overheat. According to him, he made it home, and managed to replace the thermostat. Unfortunately, the car still overheated, enough so that the top radiator hose blew, which he ended up replacing too. I ended up having the car towed to my place so I can try and diagnose what is wrong with the engine, and found some interesting symptoms.

In order to see if it is the head gasket, I checked both the oil and coolant; the oil did not have the milk chocolate appearance, and the coolant did not look milky. The expansion tank cap also did not have the tell tale "mayonnaise*. After experimenting, I found some unusual symptoms. First off, the engine was actually maintaining a normal temperature as long as I keep the expansion tank cap off. The heater core was also turned on in order to try and bleed the system of air; the heater was blowing hot. When I put the expansion tank cap back on, the engine immediately began to overheat and the heater core started to blow cold (A/C was off). I am not sure if this is truly a head gasket problem, or if there is a clog in the system. The upper radiator hose does get very hot, and even hotter when I replace the expansion tank cap. I did order a head gasket leak tester, and am waiting for it to arrive. But is there any advice that I can follow to try and solve this problem? Could it be a clog on the radiator? Something simpler? Our could it really be the head gasket going?