Please forgive this intrusion...
JCMan - I had similar problems to what you describe, albeit with a different video card. Here's something you might want to try...
Check your Vagp (AGP Voltage) in BIOS, and if it's less than, say, 1.7 then up it to that, or maybe as high as 1.8 (Do this at your own risk!).
When checking out causes of similar issues with my card, I discovered that some cards (ATI in particular) tend to be a little power-hungry. When I checked my BIOS, I saw that Vagp was at the default of 1.5V, so I tweaked it up to a recommended 1.7. Not a single CTD yet (Knocking on every piece of wood I can find, however... )
I don't know if Nvidia cards have the same issues, but it may point you an a proper direction to find a cure.
Hope this helps...
Floyd.