Reinstall your drivers but follow this method to a T. I have condensed it all into what sounds good for me, so if you want, PM me and Ill send you my version. But ive done this and its worked without a hitch.
and there is also one that is essentially the same here on OCN
The key is getting those registry remnants. Also, uninstall all overclocking software and delete their remnants from the registry too, as part of this process. I was getting problems until i decided to uninstall afterburner (problems even when it wasnt turned on or starting with windows) and then deleted the registry remnants and I havent had an issue. Sometimes you just have to . Plus, since your in SLI theres really no need to overclock the cards since you're running two factory overclocked cards anyways. Hope that helps.
Maybe if you live close to a home depot or something you could go pick up a kill-a-watt for like 12 bucks. I bought one for piece of mind on my corsair hx750 when I moved to 580's from a single 5870. I almost fell into the hype that I would need 1000+ watts to successfully run SLI 580's but now I run 2 lightning xtremes and barely touch 650 watts under full usage.