can anyone explain the surface shading? This is a new 899 on new boat. the Si & DI work great. in 2D, the digital depth is locked (2-3') and adjusting the surface clutter does not clean the shading. this makes the unit rather useless for running skinniy water here in Lousiana. My thoughts: bad 2D t'ducer, bad y-cable, or both. Any thoughts on how to fix? Called HB and they suggested updating to 7.590v. Not sure how this will help.