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.
dont know, but my 6990 is very cool now at OC 970,870... runs around 60Celsius.
thats why i want to overclock higher to 1000 or 1100 core.