There are a few things it could be, the first is your refresh rate, LCD's usually stop working if the refresh rate is too high but occasionally they go a bit mad and allow a refresh above what they can handle and you get the effect you're seeing. It may also be your drivers, try uninstalling them using driver cleaner and re-installing the newest ones, if it's neither of these it could be that something is corrupting the drivers somewhere along the line or it might be that your card is broken, nVidia cards tend to do what you're describing if they're faulty.