TR8 boiling over

["John Fleming" <joflemin@nortelnetworks.com>]

I'm having some trouble with overheating with my 80 TR8.  Only happens when
it idles, not when driving.  I replaced the thermostat and tried it again.
Same thing, after about 15 minutes it boils over.  I did notice that the fan
came on so I guess that's not the problem.  All the belts seem tight and
hoses seem secure.  It runs smoothly and no white smoke in the exhaust so I
don't think it's a head gasket.  
Anybody with a too-hot TR8 have any ideas?
Thanks,
John