Try different settings manually.. don't have that card so don't know of any good overclocks for it, try search this site or google it and you might get an idea of some good OC's for it and you could try them out..
Had to try the auto overclocking tool in CCC myself on my card just to see how it went.. all it did were setting everything to the highest and that just screwed it up..
Try 700-1150 or 725-1075 and see how that goes..
And also, imo you're not gaining that much on OCing the GPU.. it's more for fun.. so you won't miss out on any big preformance boosts if you're not able to hit a stable OC on it..