From what you're describing it seems like artifacts due to overheating memory/gpu. Have you tried underclocking and seeing if the problem goes away? Is the card overclocked? Check the fan on the card to see if it's spinning and also if the heatsink is clogged with dust.

As to the DVI/VGA difference. The VGA is an analog RGB signal so it will display something even if it's distorted and/or only displaying or two of the three channels, whereas DVI being digital can potentially black out altogether or show an out of range error if there is a fault with the display adaptor.

Crashes pointing to the Nvidia display drivers are not always an indication of a driver problem. Malfunctioning hardware can still cause this and often does. Also the fact that you're getting it at the BIOS and when booting from another OS eliminates the driver anyway.