Hmm, I ride pretty darn rough roads (especially in the spring; ouch) and the PB SuperFlash hasn't switched modes on me yet. I wonder if your button is a little too close to the plastic and it's getting set off on a bump. If this is consistent, and your case is snapped on properly, I might experiment with sanding down the button just a touch so it won't switch modes when you hit bumps.
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Work: the 8 hours that separates bike rides.