So, the inside-the-hull transducer worked correctly when you tested it during the installation process but now it does not, correct?
I have heard of some having problems with the AS-Si-DB-Y cables, but lets do some troubleshooting first:
Has the 2D sonar always given erratic depth readings?
Does this happen with everything else on the boat (all motors, radios, other sonar units, etc.) turned off?
Try connecting the HDSi transducer directly to the 998 unit and test it again while drifting. Then try the same with only the inside-the-hull transducer connected.