My Mitsubishi is overheating, tested a few things, but still no solution?

[TheWiseFool]

I have a 2001 Mitsubishi Diamante. Overall, has been a good car without too many huge problems. Im not a professional mechanic, but I tinker around with almost everything under the hood and do a lot of work on it myself. Lately Ive noticed the engine overheating while the car is idling. Also the car will no preheat if I leave it outside with the engine running. Had it started in the morning one time for 20 minutes and hopped inside and it was as cold as when i started it. The car wont even start to make heat unless driving it for 10+ minutes. Without a doubt, I went to the store to buy some coolant, but upon opening the coolant reservoir cap, I realized it was already full, which is strange because the last time I filled it was in March or April, and I dont understand how it could still be full. At the same time, I replaced the thermostat. I thought that maybe the thermostat was stuck in the closed position. Seems odd that a thermostat would crap out only after a few months, but i took it apart again and put it in a pot of boiling water, and sure enough it opened up. So i know that is still working. I did notice that when i run my enging with the radiator cap off, the coolant bubbles out. Is that normal? Over the phone, my mechanic/friend told me that it could be either a water pump issue or a head gasket issue. Is there anyway that I can narrow this down anymore and try to figure out what the problem is? Also i noticed after draining the radiator and disconnecting the hose and removing the thermostat, the coolant reservoir is still full. Is that any indication of maybe something being blocked? I was under the impression that the thermostat is what opened up and took coolant from the reservoir, but maybe im mistaken. Does any of this make sense?