The temp on my car (1991 Civic Si w/ ZC swap) flucuates. It goes way up, almost to the line before the red, and then back down to normal. It goes up and then comes right back down and it will do it repeatedly. Mostly it does it at highway speed (70-80mph), when I slow the car down, the temp goes back to normal! The car doesn't seem to be running hot and there is plenty of the correct coolant mixture in the car.

I'm thinking temp sending unit. Any ideas would be appreciated.