I have a Dell XPS laptop L502X with 1366x720 native resolution & Nvidia GT 540M GPU. Have run dual screens with a Viewsonic VA1918wm external monitor (1440x900 native) via HDMI port of laptop and it mostly works fine. But periodically, without warning (usually on a mouseclick) the system locks up. Characteristically, the monitor then displays a uniform blank screen (colour random - never the same, but fixed when it happens). This can happen regardless of me running a video or using multiple applications (I do not run games, but sometimes do 720p or 1080p video editing with PowerDirector). Nothing resolves the freeze other than complete powerdown and reboot. Any suggestions? How can I test to confirm where the problem lies? Is there a solution other than replacing the laptop? Thanks! - Ed.