Media windows highlight randomly (2nd monitor)

Hi All,

I have just bought a second monitor, running as the primary screen on a macbook pro 15'', May 2011. It is a Dell 2713H plugged in to my thunderbolt port via a mini display port cable straight in to the monitor. When any kind of media is on screen such as lightroom photograph, quicktime film, youtube window, whether it is full screen or not it keeps randomly highlighting the media, just beyond its border so that you can see the blue tint over everything, plus a bit of blue around the edge. It doesent cover the whole browser window, only the video inside. it is the same within lightroom. it happens randomly, on and off. I have tried turning off wirless trackpad/keyboard incase it was a fault (they are new as well). So far I can't find any info about the problem, so if anyone out there knows anything it would be greatly appreciated,

Many thanks,

Baz