Is your temp sender in the side of the head? I recently started having the same problem but it's due to installinging my temp sender on the side of the head(I hope!)
I had to swap covers to the thermostat housing and the one I'm using now doesn't have a hole punched in the side for the temp sender, so I installed it in the head. My guage now does the same thing you're describing. Some day I'll punch a hole in the housing and go back.
I think what's going on is that theres not much coolant flow in that area and so the metal heats up which is what the temp sender is sensing and viola!
Anyone else have an opinion?
