Oops, should've mentioned the OS: Windows 7 64bits.
I'm very wary of a clean install of Sonar, with all the addons I have. I will look into rolling back the driver version for the viedeo card, though the Nvidia software that updates the driver doesn't seem to make that any easy option. Maybe through Windows config...
Anyway, thanks for your suggestions. And JonD, you're quite right about updating the video drivers, but I suppsoe every one of us has to learn the hard way.