If the temp comes right back down to normal after the fan runs a
little, it sounds like it's working as designed. With traffic driving
the radiator loses needed air flow to get rid of all the excess heat.
In cold weather the limited airflow is made up for by the air
temperature differential. In hot weather it can't keep up, so the
temperature rises until the fan sensor turns  on the fan, which brings
it back down to normal.

> Now that the weather has finally warmed up in the Pacific NW I am
> getting some high water temps in my 87 syncro Westfalia.  During stop
> and go traffic The needle is going as far as 3/4 of the way up the gauge
> and then the fan is coming on and bringing it back to the top of the
> warning light.