Has anyone out there had this experience?

The heat guage suddenly drops to zero, and the radiator cooling fans start
up.
This occurs while driving in the winter.  The engine light does not come on.
The dealer has changed the coolant temp sensor, but the problem started up
again a week or so later.
When I called the dealer the second time, they seemed stumped.
Results of any of your similar experiences would be helpful.

Thanks