Temp Guage goes up then drops
 

I have a 1993 300sd (w140) that has started exhibiting the
following:

I drive at 70mph or so for about an hour. The temp guage
starts to rise from the normal 80-85C to 100 and sometimes
110 degrees C. After a little while, the needle jumps up a
little then drops (literally) back down to normal (80-85 degrees).

I haven't seemed to have lost any coolant. The two fans in
front don't seem to come on.

This has been irratic behavior of the temp guage. I cannot
replicate it at will (ie, drive up a hill, etc.).

Also, I haven't had any issues a lower (non-highway)
speeds.

Unless you have any other evidence that the engine is actually running at these temperatures, I'd suspect the gauge or sensor (or wiring between)