So, no answers yet, but also no solution for me. I have found one interesting thing, the distortion happen only when the graphics card is set to "External only" mode. When I set up e.g. dualhead with desktop streched to both monitors, there is no distortion.
Sometimes, I can get no distortion even with Laptops LCD screen off, but this lasts only till the screensaver kicks in, or until the monitor goes to sleep. After waking up the distortion is back. Any clues?