What inputs are hooked up and what feeds them? Do you have both COG (yellow) and HDG (red) vector lines (from memory, I think I have those right)?
I had a somewhat similar problem, turned out to be a bug in the C70 software
combined with my particular configuration (a Brookhouse NMEA
MUX which echoed bogus VHW sentences generated by the C70 back, which the C70 would then assume were valid). I had another problem when the C70 would get confused when it received the same sentences both on it's local SeaTalk
bus, as well as via NMEA
(the MUX has built-in SeaTalk
conversion). The solution to that was some filtering rules on the MUX.
Here's the response from Raymarine
support, if it helps:
VHW Is a NMEA sentence generated by both C & E-Series displays when mechanical water speed (via a DSM module or instrument received via Seatalk) is connected to the displays.
This sentence is given a false heading and transmitted over NMEA if no other heading is available via either NMEA or Seatalk.
If the NMEA receiving units, such as some position polling VHF radios do not buffer this sentence; it will be looped back into the plotter displaying both an inaccurate heading and also a skewed vessel indicator on the display.
This can be solved by turning OFF the VHW sentence in the C/E-Series NMEA Output portion of the menus.
In my case, there was a valid heading available, but the C70 would still sporadically seem to choose this invalid VHW sentence instead.
Not really sure if this is your problem, but it's similar and maybe this will help with problem determination. Easy to turn off VHW sentence output to try it!