ive heard of this before... it happens when you drive a Mitsubishi
heh im with Zeus... sounds like youve got a blockage somewhere in the system probably near the temp sensor for the fans.. so my best guess is either the top or bottom radiator tube
my buddys jeep did something similar and we just clipped the wiring going to the fan and put it on a manual switch, anytime he wasnt going fast enough to cool the radiator he'd just hit the switch and all was well... just make sure the polarity is right or you could be keeping fresh air away from the radiator