Simple: Don't run a 6990's cores at 1000mhz. That's *insane* for 24/7 usage.

Power usage will go to 500-600 watts just for the card & kill the VRM in continuous use.

Remember that it uses 450W just in the normal OC mode which is 880mhz.
It goes exponentially higher, and rewards get diminishingly lower as you raise the clock frequency.