Swap out your PSU mate.
Not that urs is bad (you got a great OC there), but Scorchio could not run his FX55 and 6800ultras fully overclocked until he changed to a 660w Enermax (then went P&PC).
For max stability in all areas that PSU proved to be the weak point in his rig.
How high can you get them at CAS 2.5?
Try pumping more volts in em an drop a fan over the top to keep em cool.
As a thought, would you consider swapping them for 2x512MB of Mushkin BH5?
Cheers
Mav