Jon Petersen

I got off the highway after 4 hours of driving, and while idling, the temperature gauge started to rise above normal temperature. Popped the hood, the coolant overflow tank was quite full. I let it cool down, and in 20 minutes I drove again with a short period of fluctuation temperature, then it stabilized.
I just barely replaced the thermostat, the radiator cap, and flushed and bled the cooling system. What could the problem be? I'm thinking it's the head gasket, but before I get myself into that project, I'd like done seconds opinions. What's your take?
Thanks for any advice.