The card is made by Asus - it is a:
Asus GF FX5200 V9520TD 128MB DDR400 +DVI +TV-out
The problem only occurs at 1600x1200, only on the second channel, and only if I use the DVI input to the screen.
Unfortunately, running the LCD at anything other than its native resolution produces severe blurring, while using the VGA cable causes fuzziness. The LCDs are both Videoseven L20CM, capable of 1600x1200@60Hz, DVI.
Thanks for your help