Can you post or send a picture of the compass and where its mounted? You probably already checked it but make sure nothing ferrous is around the compass as well. If all that is good then the next thing to try is to just run through all your dockside settings and also redo the compass calibration. If you didn't already do that...
Call Simrad tech support, they are VERY knowledgeable. They are not your typical tech support line in India, they know the equipment and can help you isolate the problem. Have you checked all the connections on the computer?
I would suggest a shout out to Wharf Rat to go over likely causes.
He knows his shit.
Is the cable from the compass spliced anywhere? You had some wonky wiring previously. Might be worth confirming it's not a bad previous owner mod.
Been on the phones me with Simrad tech support a few times.
We are pretty sure my RFC35 compass is no good.
Anyone got a lead on a replacement ?
RFC 35 long since discontinued, as is the RC25 which would work.
Options appear to be used, eBay or on the Internet.
Simrad also says other rate compasses with NMEA 0183 & 10 hertz will work.
I ha e this in my garage from the Blackfin. Took it our when I went from Northstar to Garmin. Dont know if you can use it but you can have it for postage if it will work for you. Roughly the same vintage but obviously not Robertson.