well ... i do not really know..... , i was thinking right now that sli cards need a little bit more voltage than single card configuration ..... isn't so?! what u can do to see if i'm right you may disable sli, and test stability at 841mhz with 1.025 with one card, and se whats gonna happen ...

this is the fastest way, the second way is going by lc you cards... and by doing that for a same speed you'll need less voltage