Computers are funny things. There are too many things to try and rule out, but you could try by adding 1 or 2 more fans to aid in cooling. Another pin in the haystack could be to update your drivers via the Nvidia or Ati websites.

From my experience, this does sound like a cooling issue, but my GTX295 is still doing it now and then and it's really, really annoying.

Hope this helps.

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