They could also check between the radiator and AC condensor for debris as has been previously discussed. Another possiblity is an inop electric cooling fan or a problem with the engine fan itself. if it seems to run hot at intown speeds or stop and go traffic, I would suspect the aux fan, but it's recommended to have the coolant flushed every 3 years, so if it's been longer than that I'd recommend doing it anyways. A flush and refill is fine IMHO, unless as your source stated, you had oil in the cooling system caused by an engine malfunction like a blown head gasket. Other possiblities would be a internally plugged radiator, low coolant level (caused by leaks), or simply a bad thermostat. It's even possible the temp sender or gauge is bad and the engine temp is actually fine.