get a better cable. Sounds like some kind of interference. I currently use a 15ft cable with no problems. It's nice and thick. Back in the day when I had a computer room, I was powering dual 21" monitors over 30ft of cable in another room at 1280*1024 each with no problems at all. It's all in the cable/connector quallity. I've had KVM cables 6 feet long that have the kinds of problems your speaking of.