Artifacts during GPU stress, bad card or something else?

A system I recently built for my girlfriend has had a problem with GPU artifact. The only artifacts I've seen so far are small squares, both when she runs Sims 3 and when I run Furmark to try and replicate the artifacts. A Google search came up with a similar style Furmark artifact picture here:


I have a different version of Furmark so the artifact squares are a different color but the same size and they similarly cluster around the transition areas. I'm trying to find out if this is 100% a problem with the card or possibly a different issue. Here are some of the things I've tried:

Reseated card/PCIe power cables
Reseated in different PCIe slot
Restored all software defaults
Checked that card is not overclocked
Uninstalled/Reinstalled drivers
Running memtest at the moment (no problems so far)

None of these have changed or altered the artifacting at all. The card does not run very hot during tests, the max I've seen it get to is about 80C. Can anyone confirm that this is definitely a GPU issue?

Some other info about this system:

Card: ATI Radeon HD 5870
Board: ASRock 970 Extreme3
CPU: AMD Phenom II X4 965
RAM: 4GB x2 DDR3 PC3-10700
Power: SilverStone 400W SST-ST40F-ES