Dude, those are "Vertical lines" (not horizontal) seen when there is no image to display OR with a poor video signal from a poor cable/cable connection. A good quality cable or a better connection should get rid of them, I've been told. The MK 2 puts out both an analogue signal (vga) and a digital (HDMI) signal. The D-DVI will give the best signal if your monitor accepts it; my Amy monitor has HDMI input and VGA input, so I use an A-DVI adapter, as my RTG board uses VGA and this allows a monitor switch. A poor switch will give you the same lines. When needed I can change to a D-DVI to HDMI adapter, but this means changing the input using the monitor's remote -- which I find annoying. Back to thread, a poorly shielded cable will give the same lines. You can recheck this as this was talked about on the web even before they reached the market.
I am pretty sure it is not the Amiga (they only show up with the MK 2), and it has been discussed as a signal issue -- the digital DVI will send data not wavy signals and this should bypass most shielding problems. I have OK cables myself but see these faint Vertical lines when I'm waiting for a real signal. If it were not 05:30 here and I was not typing in bed on my iPhone I would check my D-DVI output to see if even my faint lines are gone; but I can do it later if you need confirmation