As T-cut suggested, run the diagnostic mode on your dashboard to check your actual coolant temperature before deciding that it's a duff thermostat.
Press and hold the trip-reset button until it displays "test".
Press the button repeatedly until it displays "19" then wait.
It should begin alternating between "on" and "off".
When it displays "off" press it again and "test" will return.
Now press it repeatedly until "7" is displayed.
A moment later your actual coolant temperature will be displayed and will continue to do so until you either a) press and hold the trip-reset button or b) you turn the ignition off.
The temperature should continue to rise until the thermostat opens, then should drop as cool coolant begins to circulate, then begin to rise again. At some point it should begin fluctuating between your upper and lower averages. I don't know about the diesels but my ZT-T 190 typically fluctuates between 92℃ and 98℃ around town but can be as low as 88℃ and occasionally hits 100℃, at which point the fan comes on and it drops again quite quickly.
I'm sure if you post up your actual temperatures then someone can advise as to whether that sounds correct for a working 'stat on your car.
If that checks out ok then you will probably be looking at a fouled up heater matrix.
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