The needle should rest around the light area (mid-way in the scale)
after it gets warmed up.  I would suspect air in your system if this
just started after replacing the coolant.  Another possibility is the
sensor itself.  The guage has it's own little sensor (there must be a
dozen stuck in the coolant altogether--each little thing has it's own!) 
It just may be time to switch yours for a new one.

Tonya
87 Vanagon GL Wolfs
74 S. Beetle