I've suddenly been having this issue on my 1st gen Mac Pro. I'll be working, looking at the web, or FCP will be rendering, and my screens go blank. They appear to be going into power save mode. But nothing I do will bring them back. I have to hold down the power button and reboot. It is bizarre. I have the X1900 graphics card. The computer isn't crashing as all processes appear to be running. For example one of the last times it happened Super Duper was running and I could see my G-Drive light indicating that it was writing information. So i waited until it had finished before powering down and rebooting. This is becoming really frustrating because I am often rendering a complex FCP timeline. By rebooting the computer without being able to save my FCP project it loses reference to any render files that have been made. Has anyone else come across this before?