Well the mystery deepens...
I installed a new radiator cap yesterday and that didn't fix the problem (I didn't think it would but for $8 I thought I'd eliminate it from the equation). While I was at my mechanic's he looked at the fan clutch and didn't think anything was wrong with it.
He agreed that the problem is related to restricted airflow and suggested I clean out the air conditioner condenser/radiator in front of the radiator as well as it was very dirty and could be inhibiting airflow into the rad. At the moment it is encrusted with dead bugs, small stones and dirt.
I think I will try this before I tackle the clutch...I wish I had done it when I removed the rad for cleaning last time, oh well, hindsight is 20/20...
If this doesn't fix it, I'll replace the clutch (my mech has a used one for me)...