Dan, it is possible that perhaps you have a problem with the waterpump.
Sometimes the waterpumps die a horrible death and grind into the case of the
engine.  If this damage was severe enough and the person changed the
waterpump without addressing the damage to the case, then it is possible that
the pump is working fine at higher rpms, but at idle the efficiency of the
pump is effected to the point where the engine overheats.

The other possibility that comes to mind is that you are only experiencing a
gauge problem.  In other words is the engine actually overheating or it is
merely the gauge that is reading high?  It is possible that the sender for
the coolant temp has gone bad, or the voltage regualtor on the rear of the
instrument cluster is messed up and causing a high temperature indication
problem.  Hope this helps.

Thanks,
Ken Wilford
Van-Again
John 3:16
http://www.vanagain.com
Office: (856)-765-1583
Fax: (856)-327-2242