RorrKonn my Dell has nVidia graphics. I am using it as I type this. There is another Dell XPS on the desk in the bedroom with one as well. Don't speak with authority if you do not know of what you are speaking.

Boni you may be getting a TDR failure. Widows by default is set too low, about 2 seconds. For some intensive calculations this may cause a screen reset. Really intensive Superfly calculations can take too long to solve and the video card times out and resets. There is a thread over at the SM forum from some time back and possibly one here as well. A change to a material or a render setting can cause this in a scene that rendered fine previously.