Furuno FI-50 & FI-70 Troubleshooting


So due to parts availability in Trinidad, we've ended up with a depth sounder system that has an Airmar tridata transducer and two furuno displays - one FI-50 and one FI-70.  There is nothing else on the NMEA 2K backbone except for a power supply.   The FI-50 keeps resetting its calibration for depth, and I can't figure out what's prompting it.  It does it every time I power off the system, but but it also seems to reset at random while being on - to the point where I can re-do the depth offset 5 minutes before entering a harbor, and then check the display as we're motoring in the channel and it has reset itself back to what seems to be its default, -.1m. Any thoughts on what to do to make it save the offset calibration?  I've applied a speed calibration as well, and that seems to be sticking.  The FI-70 has no problems other than it won't stay at whatever brightness I set it at - it likes to default to a brightness that's hard to see in daylight after a few hours.