Won't stop heating up.

[evan]

I drove my truck for the first time in about a week today, and the engine wouldn't stop heating up. I had the heater cranked but it was just blowing cold air. I shut it off and let it cool down and then started it again. The second time it looked like it was going to keep heating past the normal 1/4 of the temp gauge, but then it started blowing warm air through the heater vents and instantly dropped down to the usual temp.

Does that sound like a bad thermostat to you? If it does, what temp of thermostat should I replace it with for normal operation? (It's a .030 over 350)