Sometimes it's difficult to get the SI xducer low enough on the transom to actually be in the water as the boat comes up on plane ...
Now the issue with erratic depth readings from the shoot thru could be several possibilities ...
If the erratic depth readings are only at higher speed ...the issue is most likely the mounting position of the shoot thru ...
If the erratic depth readings are at all speeds ...the issue could be the mounting positing, the epoxy failing, the AS SIDB Y cable going bad, or a failing shoot thru ...
Is your depth reading issue at high speed or any speed ... ??
Rickie