Diesel running hot.
 

I have a 84, 300sd which is now running 20 degrees hotter (at 100). Replaced thermostat, radiator ok, no signs of oil or bubles in expansion tank, hoses ok, heat works, v-belts ok, fan still there. Noticed that the temperature guage does fluctuate from time to time, as does the fuel gauge.
What do you think?

if every thing checks out then could just be the gauge it shouldnt bounce around like that.

If the gauge is correct, it could be a bad fan clutch.

Under what conditions is your car running at these temps? Also, is the fluctuation in relation to speed, where temp goes down on highway and up in traffic?