Diagnosing a fault with an Autohlem ST50 Plus Wind Instrument

[meshugah]

I have an Autohelm ST50 Plus Wind Instrument fitted to my yacht which is based in Port Douglas, Queensland, Australia. The wind gauge pointer flutters all over the place even in a mild, steady breeze. Is there an easy way to determine whether the masthead unit is faulty or whether the fault is in the gauge itself?

Any assistance would be much appreciated.