Bill, it sounds like you have pretty much the same system and transducer location as me except that I have separate transducers for speed and depth. I regularly get depths to 300'and often to 400. But it's really frustrating when I'm looking for just the right place to drop the hook and the bottom is at 15' and rising and depth sounder goes out. I suppose it's possible that it happens in deeper water too and I'm just not paying as close attenetion to it, but I don't think so.
I'm not sure of my exact transducer model number, but I was just looking at the Airmar site and see that tranducers that look just like mine are internally compensated for various installation angles. So the angle probably isn't the problem.
Maybe I'll just replace the transducer insert and see what happens. Fixing this is on my priority project list for this winter.