Fluctuating temperature gauge. Sometimes

1996 VW Jetta Wolfsburg, 2.0L. 90000 miles.
Recently my engine temperature gauge has been
behaving oddly. Normally the gauge goes to
the midline (190 degrees) and stays there.
But if I drive the car more than 40 miles, the
needle starts to creep up past the midway mark
and then moves around erratically. The water
pump was replaced recently and there is no
coolant leak, nor any other symptoms suggesting
that the engine really is overheating. The needle
has never gone all the way to the right; it has
gotten as high as 230. What might cause this?