I just wanted to bump this and let you know that I think I may have solved the problem, and to thank you all for your input. I know you have been waiting all summer to find out how this ended!
I have an electronic temp sender unit. The sender location on the manifold is a 3/8NPT thread and the sender itself is a 1/8NPT. This means I use an adaptor, like this.
But, (and here is where I think the problem resides), because of the thread taper the adaptor doesnt fit all the way in to the location and the sender doesn't fit all the way in to the adaptor. The sender itself is barely touching the coolant.
So I cut off the final half inch of the adaptor, effectively lowering the sender into the manifold and deeper into the coolant. Result? The engine (apparently) runs much cooler when the car is moving and peaks at about 185 before the fans kick in and they now (apparently) bring the temp down just as they should.
In fact, with the 160F stat in CT in november the whole thing runs a little too cool, so I may go back to a 180F stat for the winter. Also I have ordered a new sender with a 3/8NPT thread so I can eliminate the adaptor altogether.
Yay!