Well Thracks thinks its simply down to a not stable overclock. I'm not sure what to make of that as it does it from 950 all the way up to 1100. Find it hard to believe that running on everything else fine it isn't a clocking card at all. Would those of you who have also had this issue believe that? No disrespect or anything, he will clearly know a lot more than me!
Does it do it at stock?