Could be the mast unit. When you turn through the wind does it suddenly
jump after it has been sticking?
There is a pot.meter inside with a magnet on it, it follows the magnet
thats in the windvane. The pot.meter tends to wear out after a while.

Or can you see the pointer of the instrument thouching the glass of the
instrument? If so, gently push on the middle of the glass(plastic) to
push it back down on the motor axel.