After running for a few hours, the video monitor starts flickering with coloured lines across the screen, eventually the screen goes empty with a 'No signal detected' message and then the monitor goes into standby mode. The PC still has power, the fans are still working, but no disk activity can be seen and it does not repsond to mouse or keyboard activity (e.g. CTRL-ALT-DEL). I have re-seated by video card and installed the latest driver from the AMD.ATI site.
Once this behaviour has started, the machine will only run for 10 minutes or so before the behaviour repeats.