Same sorta thing happened to me once. I was driving home and was just stopping at a red light. My thermostat was good, and while sitting at idle it just climbed like crazy. Light turned green just after it started climbing, and I was going to head for the nearest place to stop, but just after I started moving, it shot back down to 210. Then it just kept doing that...climb climb climb climb...DROP! Sometimes it would stay up a little longer, and I'd blip the throttle or something to try to shake the car, and it'd drop. I don't know if that actually helped it or not, but I was doing whatever I could to make that last mile.
The next morning, popped the radiator cap and turned on the engine and watched the level. Topped it off every time it started emptying (in other words, refilled my radiator). After that, never had the problem again.....except when my power steering cooler cracked..but that's a whole nuther story.