The thermostat could be the culprit.  Also if you system has ever been treated
with the dreaded "Stop Leak" then your radiator could be gunked up.  When you
flush your system I think it will become apparent if anyone has added any of
this nasty stuff to your system or not.  Also check the operation of your
cooling fan.  When does it come on and when does it go off?  The temp sender
for the fan could be faulty.
Hope I gave you some ideas.
Ken Wilford
John 3:16