VW V6 overheating mystery

My 2001 Jetta VR6 with 50K has been overheating under load only - OK while idling in the driveway. When I drive off, gage moves up and alarm comes on. Mechanic pressure tested for leaks, coolant circulates into reservoir (while idling, anyway), and fans work. Coolant in system is the correct type, too. We are at a loss what the problem is, and I would like to avoid tearing the system apart needlessly.