If you are doing an infrared heat check, see if there is any cold spots on the radiator, especially directly in front of the viscus clutch-fan, as this seems to be quite common, in which a cold spot there would fool the clutch-fan!

 

Mystery solved--it was the radiator ! When my mechanic first took out the radiator he
gave it a cursory look, cleaned the fins with a hose, ran some water through it and
thought it OK. But fortunately, he thought about it a while and talked with other mec-
hanics and decided to send it out. It was a Behr radiator that was installed in July '04
but out of warranty. I had a similar thing happen with my GMC truck--the replacement
radiator wasn't a year old and it clogged up.
Thanks for all your help.