jfoj: regarding the t-stat in the bottom of the expansion tank, I replaced that when I did my wife's cooling system overhaul, so that is relatively new as well.
OK so I did the test leaving work yesterday. It was about 40-something degrees out or so, when the car started up, the temp read 23C. I find this pretty odd considering that translates to 73.4F? Maybe the coolant never gets below a certain temp? seems odd though.
Anyways, I started my car at 1:11pm, and around 1:15pm, my needle was dead center, and the temp reading was only 75C (so this shows you how much temp variation there is when the needle is dead center!)
In less than 10 minutes total time, my car was up to 95C and steady there the entire time I was driving, saw it go to 96 once and down to 94, but it stayed pretty steady at 95C.
I replaced my thermostat when I did the water pump around 100K I think. I am now almost at 170K.