Over heats at idle cools off while driving
 

99 Chevy blazer 4x4 4.3l vortec. Over heats when sitting at idle for a few mins and cools down after driving for a little bit.

not exactly sure but maybe it could be a bad sensor? or maybe the fan kicks in only when you are driving? Just a thought

Check out your fan clutch. If the blades are free-wheeling with the engine off, then that could be your problem. Fan clutches do go bad occasionally. overheat at idle is a common symptom.

Or a partially plugged radiator