Autohelm ST5000 Wheelpilot problems.

The unit seems to work fine with its fluxgate compass sometimes, but
'throws wobblies', in which the display goes berserk - displaying
moving random segments of numbers including the small 'Cal & Wpt'
writing at top right. Rapid de-clutching and manual takeover is then
needed! Since the power supply wiring was way below the makers
recommendation (.75 sq mm over 5m run) I've rewired it using 2.5 sq mm
in case the voltage drop with the drive motor running was causing the
electronics to 'drop out'. This 'cured' it for about 20hrs use, but
now it's back to its old tricks again. Resetting to 'Standby' results
in the display going to 'flashing C 060 ' (regardless of heading);
repeated stabs at Standby can get the heading back to whatever it
should be, then 'Auto" sometimes works for a while before the next
'wobbly'. I've been through the complete calibration sequence - which
displays perfectly - including the '3 slow circles'.

Does this sound like the fluxgate is at fault, or the actual head.
Both have been professionally checked out before re-installing (the
cabling is a real pig to get at), but then it is an intermittent
fault. I couldn't get any sense from it using the NMEA input with the
output from a NASA Clipper Wind either, but that may not be part of
the same fault!