Well I have only seen something close to that once before, mine was split horizontaly and not vertically but same idea. I was not tethered though, but it ended up my memory card was bad (the only time it happened is when I was shooting in continuous High, FPS). It worked fine when I only shot one picture at a time and had a 2 min or more break between shots. I suspect it was the card starting to fail but don't know for sure (I tossed it once I saw the problem).
In your case I might look at the cable and maybe the hard drive on the computer your using (this is where I would start but honestly have no clue). Hope you find the problem.