I was on the water this weekend and everything worked just fine, then when I swapped over from map and sonar to SI the unit plays maybe half an inch, stops, then starts for another short line, and does this repediately off and on, I couldn't ever get it to go back. Everything else on the unit works just fine. It's an HDS 8 gen 2, I bought it last summer so it's fairly new, and I've never had any issues with it. Any ideas of what could be causing this?

I have the issue with the SS finding bottom occasionally. I force a depth on it rather than auto, have to do it for both side and down imaging. When it starts rolling again, I go back to auto. This usually happens in shallow muddy water. It is a quick fix, and works on 2d when the bottom is lost too. If all else fails, I will gun the engine in reverse to run a cleaning stream of water over the hull and 'ducers.

I'll try that when I get back on the water. Anything else?

I had a similar problem. Of all things it was a bad ground to the battery. The nut on the battery backed off just enough in rough water, it was causing problems with front and back units. One would work but if you had both on it was trouble. I pulled the leads and cleaned them and tightened everything back up when I hit dry land. Never had the problem again. Not sure if this is what is wrong with yours but it will be an easy thing to cross off your list.