95 Ford Probe overheating?

I just bought a 95 Ford Probe GT with 85,000 miles, with a new transmission and radiator. The car was vandalized and needs some work cosmetically so I got a good deal on it. However on my drive home I noticed that the temperature gauge was creeping above the normal range whenever idling, however before it would get too hot I'd hear a sound like popcorn popping, and the temperature would drop into the normal range again. Knowing the trick of turning the heat on full blast when the car is getting hot, I tried that to discover that the heat doesn't come on. I managed to get the 10 miles home by taking almost an hour, and pulling over whenever it started to get hot.

Anyways, I'm a college student and am only just starting to learn about cars, and my plan is to register the car just before summer, and take it to my mechanic to have everything mechanical fixed with it. I've been led to believe that this is just a bad thermostat, but I'm concerned it's something more. I don't think it's the head gasket, since the head gasket in my last car (1995 Pontiac Grand Am GT) blew twice and this is completely different symptoms. There's no steam or smells like when the head gasket blew in my last car. Anyways I guess my question is, is this problem really just the thermostat, or if not what else could it be and about how much repair cost am I looking at?