Why Did This Happen?

[EMPiRE]

It looks like a hardware problem, try to re-insert your graphics card a few times, when the power is off, into the agp slot to make sure that it is making good contact.

If it still happens make sure that the graphics card is not overheating (make sure the ven is working or add an additional ven, point it to the graphics card and check if the problem still occurs

If it still happens check the computer setup and check if it is possible to change the voltage given to the agp port, if there is a setting up it a bit to see if the problem goes away.

Also to make sure update your graphics card drivers to the latest version (just in case it is software related)