My 06 Mega Cab CTD takes at least 15 - 20 minutes to get even close to warming up, no matter how hot or cold it is outside.
Once it is warmed up the temp gauge will stay at slightly under 200 if you keep the RPM's at around 1800 - 2000. If the RPM's drop at all below 1800, the temp gauge drops at least a half inch below the 200 mark. To me this looks like a fluctuation of about 20 - 30 degrees. I have never owned a diesel vehicle where it didn't reach operating temperature in a few minutes and stay there. The temp fluctuations and the amount of time it takes to warm up can't be good for a diesel engine.
Do I have a bad thermostat, a bad gauge, or is this normal ?

 

The cooling system on these trucks isn't even CLOSE to capacity even while towing, so when the thermostat opens it drops the temp FAST. Mine will fluctuate 20-30° just while driving. Taking 10 minutes to warm up is normal. On a hot summer day (like now) I get about half way to work (~8 miles one way) before my temp is up to 174°. In the winter sometimes the truck is JUST getting to the 174 mark when I reach the gate at work.