Sync gauge

Ian

FV310 2003. Anyone seen this and/or have ideas? Until a week or so ago no problems but then slow cruising the sync gauge suddenly heaves to full starboard and no adjustment, turn off and on again corrects it. Back at dock after awhile, the same but later all good. Yesterday, all fine and then starting up after an anchoring session, same again. Where is this fed from and anyone have any ideas?