No Image on Secondary Display of MBP Help Needed

Hey All
I have a MBP that I purchased in 2006 hooked up to a BenQ 19inch external display. I have the MBP extending the image to this display. It has been working fine for the last 2 years but yesterday all of a sudden the screen went black. I can still drag windows onto the secondary display and Mac OS X recognizes that this display is connected, allowing me to change resolution etc, there is just no image appearing. If I unplug and then re-plug the display into my DVI port an image appears for about 1 minute before going black. I have used the display on boot camp and this is not occurring so I'm assuming it must be a setting in Mac OS X.

Any help would be greatly appreciated,

Luke