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!

The alarm is built into the Sapphire graphics card itself so not really an Asus probe or alarm. I'm a bit concerned about frying the card as I don't know if the alarm is (erroneously?) signalling over or under voltage or lack of amps.

It has been suggested that I try the card in the other SLi slot but I understood that using just a single card in the second slot worked with less performance.