HDS 5 I think Gen 2. It was updated to the 4.1 software when I installed the new Navionics card about a month ago. I am pretty sure it's the unit itself as I am having issues with the sonar as well. I put a new ducer on this weekend and it isn't working properly. I have the same unit on the bow. I'm going to swap them out and see if it works any better.
Let me know how that works after you swap the units, it does sound like it may be a unit problem, but if for some reason it is the card just let me know and we will get it worked out.