ST40 Bidata & C80 both showing 3.5 ft wrongly

[danstanford]

Depth has been fine till 4 hours into a trip when the depth changed to 3.5 ft despite what the depth might be. I played around with it to no effect till I gave up and started plying around with the data bar on the C80. When I added depth to the data bar all of a sudden the ST40 started showing an accurate depth as did the C80 data bar. All was well for about 20 hours till when I added a waypoint I noted that it listed 3.5 ft. This caused my to look at the Bidata which still showed the correct depth then all of a sudden it showed 3.5 feet again as does the c80 data bar.
I have tried all the combinations of steps without any effect including powering down. Does anyone have any idea why it is locked up?