[bluebrit]

My first guess would be to see if your video card is over heating. If so that could trigger a shut down to protect your pc from further damage.

A basic test for this is to try running a video with the case open so more air gets in and see if you have the same problem.

If there is no temp diagnostic for the card that will tell you the running temperature, carefully put your hand on the heatsink over the cpu and see how hot it is.

Before doing anything inside the case, touch the casing with your other hand, to make sure you are grounded and have no static in your body.

Video cards can get really hot hence i say carefully. If it is getting very hot, it can affect the card and the components surrounding the card that are directly on the motherboard.

Make sure the fan on the video card (if there is one) is clean and running freely and make sure the air holes in your casing and around your cooling fans are clean as well. All of the above can very quickly ruin a pc if not kept clean.