Mmmmm yeahhhh....
You're OCing too far. Back down on the core and/or the mem clocks, and it'll be stable.
You'll more than likely find that clocking it down another 10MHz will make it stable for ~15-90 minutes, but will eventually crash again. Drop it another 10MHz and you're probably golden, but of course this is subject to testing and finding the limits for your specific cards.
I followed your advice and was able to get 760khash @ 1800 memclock and 1120 engine. Should i OC one card at a time or it doesnt matter if i oc them both at the same time? And i am using 1 card for display, does it affect the limitations of oc'ing?