It's very clear that the OP has problems with the unit as the behavior
that it exhibits is not normal. Seeing the boat position jump around
makes me think that the GPS antenna (or whatever is providing the
positiion data) is at fault. I can imagine that if that's the case,
that the repair facility can't find a problem with the C120.

It isn't clear where the video was shot, but if the video was shot
while at anchor, then you can easily understand why the radar overlay
of the charts isn't in sync as the GPS can't determine the course
while not in motion. This will cause the radar overlay to do exactly
what is being shown on the video and makes me believe that the author
doesn't understand how the overlay (or chartplotters in general) work.

I will agree that the user interface can be a bit confusing at times,
but the video certainly makes it appear that the owner hasn't learned
the interface. His complaint could be aimed at any manufactures'
unit. It's really up to the purchaser to do their homework before
purchasing the unit to determine if they like the interface. You can
walk into any Westmarine and play with the RayMarine units as long as
you want, so I don't have a lot of sympathy for this complaint.

The software on the C120 allows you to view/log the SeaTalk data and
see error counts. I would concentrate on this and get some
professional assistance to debug the system. Remember, all of the
units on the share a common bus, and any device on the bus can
potentially cause problems.

-- Geoff