Close all background programs -- the GPU will not run at 100% load if something else is running (for instance, a web browser). Use -f 1 argument when using miner and see if it corrects.
Sometimes, cards also behave somewhat erratically as far as efficiency for a few minutes of putting them under load and then chill out @ full load.
Alright, I'll throw that in.
It could also be (probably less likely) that your card is over-overclocked and the driver is resetting every few seconds.
Nah, with this card it's really obvious when I do that, the screen flashes black and resets and then the core clock resets to 400MHz and won't change until I restart. I will turn some of the clocks down though.
Later update: the -f 1 option worked, thanks!