"No data" message on st6002

[Art20c]

Hi all. I have an autopilot system which consists of st6002 control head, fluxgate compass, st1 smart pilot computer and st4000mk2 wheel drive. Other instruments are st60 speed, depth and wind. Mfd is es75. All connected together via seatalking using seatalk to seatalking converter kit. All works fine, except times, happens kinda often, when st6002 head displays "no data" message while autopilot is engaged. Setting pilot to standby and back to auto does not fix the problem. But after pilot is set to standby, "no data"message disappears immediately and head is showing its routine screen with heading and rudder bar. "No data" happens in both, heading and wind wang mode. This might happen several times and for several miles, then everything is back to normal after pilot is turned on. Could last for hours and many miles until it happens again for no aperent reason. All connections were checked, found nothing wrong. What could be the problem?