I've had radiator problems before with this car... had hoses replaced and such, and things were OK for awhile. Lately, it seems to be heating up randomly... sometimes I drive 10 miles to work, and the engine/radiator are both extremely hot when I pop the hood afterwords. Other times, I can drive 50 miles, and the radiator cap is not hot at all afterward. The coolant temp gauge never goes past the halfway mark, and there doesn't seem to be any leaks. What's the problem?