|Quite a few systems were sold with G31 systems and 9500GT cards.|
Did you bother to follow instructions for uninstalling the onboard graphics and installing the 9500GT? Or just shove it in and hope for the best?
What is motherboard manufacturer and model number? G31 is just the Intel chipset. What do you have for a power supply and what is the amperage rating of the +12v rail?
The 9500GT does't require a heck of a lot of +12v power but the extra 3 amps it needs could be a factor here.