I had that problem with a card once, it turned out to be a faulty card.
I was able to get around it for a little while by overclocking my processor. AMD Phenom Quad core.
Using CrystalCPUID

The Program is CrystalCPUID (http://crystalmark.info/download/index-e.html#CrystalCPUID)
You have to run the CrystalCPUID and then run the TLB Which can be downloaded (https://rs90cg2.rapidshare.com/#!download|90cg|99264216|PhenomTLB_v_1.04_with_built_in_DLL.rar|264|R~0)

I don't know why that worked but it did. There might be a version out for yours.

I replaced the card as soon as possible, if yours is under warranty I would look into that.

-Keichtech