That is bizzare, I had the opposite problem with my GTX 570, DX11 crash to desktop on Heaven and DX9 stable for hours. When you went up to 1.28, were the temps noticeably higher? If not, just stick with 1.28.
FWIW - Microcenter has the Sparkle GTX 480 SXX4801536D5-NM in their ad for 239.99 after rebate. Not sure if you have one of those stores by you, seems like a fair deal if you have they have one in stock, my local store had four until three days ago.