theres a problem, don't leave it alone!!!
could be a lazy/broken water pump, clogged rad or air in the system.
also have a fiddle with the earths, mine used to go to red on occaison, till i nudged a wire while looking at it and the wife spotted it go into the red. i attached a few more earths and it sorted it.
i suspect yer pumps ferked myself tho!!!
have you got the correct 50/50 mix in there?