Hate to let this on, but you must make one change at a time, bios, driver, dx8/8.1/9, vid properties, oc'ing, whatever. And then bench every single change to see wether it helped or hurt performance. Try the card with only a slight oc to see if there's yer prob. Then sneak up on the oc till you drop off, then back up a lil.
Very easy to spend a day doing this.