I had a similar, but not exactly the same, problem with mine. It was related to NVidia's NView software (have you activated it).

The problem was that NView is not really designed to cope with screens that can rotate and in my case I had to disable it for when the tablet is not connected to an external monitor. There are a number of steps to follow to check/set this

To see if nView is enabled and causing problems, right click on the desktop and if you see NView Properties in the context menu that appears click it. The Nview properties window will spring into view. Simply uncheck the "enable nview" option at the top and away you go.

Hope that helps - that's all I can think it could be.