Erratic Heading readout display

[jgonnerman]

System is an e125/ smart pilot/st8000 control head/quantum radar/raymarine gps receiver

While on a recent coastal trip while heading roughly 320-340 magnetic the COG was steady. During the roughly 90 miles the heading value would vary 60 degrees both east and west of COG and then return to a normal 2-4 degree difference from COG. This deviation would last long enough for the radar overlay and radar screen (MFD was in heads up mode) to reorient. COG was stable and autopilot did not respond to the deviation shown on the MFD. This behavior did not affect the steering input of the autopilot when in auto or track mode. The autopilot/depth/gps are feeding the MFD through a seatalk ng converter. The depth and position readouts did not display this erratic behavior.

For return trip heading the was reciprocal course of 140+/-. The heading readout was stable for the return trip.

What might be causing this erratic Heading readout.

boats motion didn't seem to show on the autopilot control head even in standby mode readout was stable on control head.

bad connection wiring?

Something in the MFD

Thank you