You shouldnt get more than 340 mhash on a 5970 core @ 725 MHz, even with perfect ram speeds and miner/kernel options. If you're spiking 350-370, definately something awry going on there. Is your GPU activity maintaining 99+%? Doing nothing on the computer running on the miner? I'm running 5830, 5870, 5870, 5970, and 5970 and I get sudden dips of GPU activity (40-70%) if I do ANYTHING on it, and it kind of cascades between the 7 GPU's for about 30 seconds before stabilizing and getting back to 99.5+%