Its a realtively new PSU (650W 12v/40A supply) but I will check the voltges anyway next time it happens. I don't think it could be anything to do with windows or software code. The monitor fails to display the BIOS messages at boot-up or the OS selection screen to select XP/Win 7 partion - no OS system is actually loaded but no display.
I will return the GFX card just as a matter of elimination. It may be something to do with the GFX BIOS and Nvidea Adapative Power Management but I'm guessing. It seems odd that IL1946 works perfect but CloD is a total mess - that seems to indicate that the GFX card is not adapting to the new GPU loads placed on it unless I drain power from the Mobo by switching the PSU off and restarting all the BIOS routines.
Not to worry, if the problem remains with a replacement GFX card installed I have the perfect excuse to upgrade to a i5 2500K..............
Thanks for all the help and interest guys!