Don't feel bad i ran a GTX 580 before i had any ATi cards, it was a MSI lightning too, real power hog for 160MHash/s, I never ran into the issue, in fact i think the powertune feature looks more like this:
GPU Clock: 950 - 880 - 920 - 880 - 920 etc, rather than usage, the usage issue you're having sounds more like CPU affinity, try selecting a different CPU core, or more than one, you might even want to change the flags to something like:
-w 512 -v
-aggression=4 -gpugrid=64 -gputhreads=384
Depending on which miner you're using one or the other will work. See which one gives you the best results.
The 6950's i have are the one's that have powertune, or whatever where i have to increase the power setting slider up to 20% in CCC to stop if from fluctuating, my 6870 does not do this at all.
-Also shata, thanks for Hi-jacking my thread!