Two things can happen to this indicator, first there is a modification kit that King issued to stop the problem of sticking GS bars.
Second there is also a recomendation that a capasitor in the unit is replaced to stop the GS indication from becoming voltage sensitive.
Dont go digging around in the panel untill the HSI has had a looking at, I would recomend the avionic workshop at Stapleford for this but dont go to the company's head office in Lingfield because they know very little about GA avionics.
Oh ! dont try Mad jocks idea the polorised glass in the front of the unit costs £200......................... you can guess how one of my customers found that out !