2000 chevy tracker is overheating!!!?

My 2000 chevy tracker temp gauge goes to red SOMETIMES when I am stopped in traffic and usually goes back to normal when I drive forward again. Other times when I'm stopped it stays in the normal range. It seems to be random and when I turn the heat on to try to bring the temp back to normal it doesn't seem to work. The only thing that brings it back to normal is moving forward. It hasn't overheated when I am driving (so far). I just bought this car about three days ago (not knowing it did this) and the radiator and water pump were supposedly already replaced. We replaced the thermostat last night and it might have been a bad one because I had no heat and it took forever to warm up. When it finally did and I drove it, the temp gauge went all the way to hot and stayed there. There is no white on the oil dipstick so I don't think I blew a headgasket (yet). Any ideas? Is this a common problem for this year and model car? If I fix it, is there a high probability that it is going to happen again? I need a dependable 4x4 for college so if this isn't going to be it, I need to get a new car ASAP.