Thiev: Try disabling SLI / Crossfire.
Failing that, please post your system specs.
OK, disabling SLI did resolve this, however it seems I had to do it at the system level. (I would much prefer setting up a games profile with the Nvidia control panel, but the options I set for single GPU didn't seem to help).
This is the first time I've ever had to disable SLI for a game. I'm hoping that isn't the permanent fix.
Thanks for your help.