CRD with 98,000 miles.
All winter temp gauge has been hanging around 1/4 warm. So, I'm thinking thermostat is shot. Today, air temp is around 40 degrees and temp gauge goes to 1/2--right where it should be.
Am I looking at an occasional sticking thermostat or some other problem?

Yeah your tstat is stuck open like most of them get, and the warmer weather helped it out today. Looks like you are do for a timing belt soon, you might as well get a new tstat and water pump while you are tearing things apart.

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2006 LTD Bright Silver loaded with all the needed mods, CCV intact.
Proudly supporting CRD vendors, and their development of quality parts and accessories.
Equipped with HDS thermostat, plenty of heat, faster warm-ups, increased fuel mileage.