-f flag boosts my MH/s for a couple seconds...then falls flat!

[Stiletto]

So I see these people using the -f flag, which makes their rates skyrocket.

I've got a Radeon 6970 running using GUIMiner/poclbm, oc'd to 950 with +20% power control and the fan keeping it at 75 or less. Pulling 380-390.

-v actually HURTS my rate
-w128/256 doesn't seem to affect it
-f momentarily sends my rates up about 70-80, then it crashes back down to 380-390

Anyone else with a 6970 care to share their flag setups and their results?