I've come across a machine that did this recently and still haven't resolved it yet. It's not a monitor problem (tried it with two different ones) and can't be the graphics card because you've tried two. I was talking to someone recently who had the same problem and don't think it's the monitor cable either (but could be). This leads me to thinking it may be a motherboard problem and, since yours is new, probably nothing to do with dust. My conclusion is either a duff motherboard or an overheating problem, but it's only a guess.