A8N SLi Deluxe with ATI 2900 Pro power alarm (?)

Asus A8N SLi Deluxe - new Xilence 700W
Upgrading video from Asus EN7600 GS to Sapphire ATI 2900 PRO 512Mb - works fine except in full screen mode.
Uninstalled all NVidia graphics drivers via control panel and EN7600 via device manager.
Connected card to motherboard & used 2 x 6 pin PCI-E power cables for Xilence 700W ATX 12v V2.2 power supply.
Installed Sapphire card using installation disc. Updated drivers from ATI to 7-10_xp32_dd_ccc_wdm_enu_53250.exe
Card works just fine until you run anything (ie game) in full screen mode at which point the card alarm sounds continuously to indicate a power supply problem. Come out of full screenmode & everything works just fine.
I have also connected the EZ-Plug to the motherboard without effect.
If I install the same card in my Asus A8N32-SLI Deluxe board (similar peripherals spec but only 480W Xilence power supply) the card runs everything you care to throw at it full screen or otherwise.
I have tried eliminating the power supply by swapping the 480W from the A8N32-SLI Deluxe board into my machine but still the same problem. I have re-set the BIOS to factory defaults but still the same problem. There is no overclocking or overclocking utilities working here. Help!