I have the same problem with my 595c and a friend of mine has a 700 series that he is having the same problem with. :'( :'( :'( I TROLL FOR KOKANEE here in California and depth is very important. I can correct it by turning it off and on, but then I loose my waypoints and track. If I do nothing it eventually corrects itself.
I am going to tryit on auto depth and default and see if it makes a difference. I won't be able to try it until late next week though.