Sorry, this is going to be a bit wordy. I have lots of details...
A few weeks ago, the temperature was in the single digits with a negative wind chill. I started up my 96 Blazer and let it run for 10 to 15 minutes. I then drove around the corner, and by the time I got a few blocks away, the temperature was at 240 degrees. I immediately pulled off to my uncles house and let it cool down. After looking at it for a while, I drove it back home and then to the store. On the way to the store, it would get up to about 230 while idling at a stop light, but would immediately drop when I started moving. Anyway, the next day the temperature warmed up and I drove it from Chicago to Indianapolis with no problem.
Well yesterday it got to 15 below here with a -25 wind chill. I went to start it and it started OK. After warming up for 20 minutes, I decided to drive it around the parking lot. I got stuck in some snow briefly (even with 4WD), and by the time I got out of it, the temperature was 240 degrees. By the time I got back to my spot, it hit 260 and the radiator cap released and blew fluid everywhere. (No fluid in the oil.)
I waited until today, put some more anti-freeze in it and started it up. No problems at all. I drove around the block and it got up to 230 again. I let it idle for a few minutes and it stayed at 230. Finally, I hit the gas and got the RPMs up to about 3500. The temperature climbed to 250, and as soon as it 250, it immediately dropped to 150 (within seconds). Since no fluid was lost and I didn't crack the block, I'm assuming that it finally decided to cycle the fluid through the engine, which is why it dropped so quickly.
Anyway, the only hunch I have right now is maybe the thermostat is stuck, but given that this only happens in extremely cold weather, I'm not 100% sure. Does this sound like a problem with the thermostat?