2005 Honda S2000 OVerheating

Question about a 2005 Honda S2000. For the past 5 years there have been no mechanical problems. Two days ago it randomly overheated in the evening (it had been fine all day). It wasn't blowing any heat into the cabin either. I immediately shut it off, let it cool, and then drove it one mile to my buddy's garage. They changed the thermostat and claimed that they checked the water pump and that it seemed fine. However yesterday evening, after it had been fine all afternoon, it spontaneously overheated again. Neither time was a slow process either, the temperature gauge just shot up really fast. This time though, I had to wait a minute to pull over and in that time, the temperature went up really high, then dropped back down to normal, then went up really high, and then back down to normal. I pulled over several times on the way home to let it cool (it was after midnight, I had no choice) but the temperature kept jumping up and down like that. It did it at least 5 times and I only drove like 5 miles to home. Keeping the revs up at first seemed to help cool a little bit but then it would just get hot again. We can't figure it out what is wrong with it. Help!