How is the coolant level in the expansion tank? Frequently, when this
level gets to a critical level( for the level sensor)the light and needle
can to haywire,occasionally. The level varies from when driving  and when
idleing. Cold and warm. The needle goes to hot when the light comes on.It
does not mean you have overheated.It will scare the bejesus out of you the
first few times it happens. I am supposing that you need to look closely
at your coolant level and moniter it often.If you have begun to loose
coolant, you can claim membership in an exclusive Vanagon club.