Once again I will state that you need to look at the system as a
whole. The display head my not have any problems, but there could be
issues with devices on the SeaTalk bus. This possibly includes a
SeaTalk GPS antenna and any other instruments.

I have a C80 which runs the same software that is on the C120 and I've
never seen any of the issues that you describe. The only difference
between the C80 and the C120 (that I know of) is the size of the
display. Since RayMarine has already replaced the system board on the
display, this makes me believe that the problem lies elsewhere within
the *system*.

Who installed the system for you? Have you had the installer out to
figure out what's going on? What other devices are in the SeaTalk
bus? Have you reviewed the output of the diagnostics such as SeaTalk
message errors? As far as RayMarine tech support, I have always found
them very reasonable to deal with if you approach them with a
reasonable attitide.

-- Geoff