Just to throw my experience with these cards in case it helps.
My reference 5870's I run at (925-150-1.07) @ 425 m/h @ 180-200watts...different cards, different personality
Ya my reference 5870 runs at 925-300-0.953 for 425MH/s @ ~160W. Those lower voltages make a big difference!
I wish i could get my 5870's to run at that voltage, but they tend to crash out at anything lower than 1.07 at that engine speed.
I tried your timings on my 5870's, the first one seemed to take and I almost ran downstairs to see how much the wattage had dropped (Rigs are in my finished basement), I saw a huge drop was excited and then I just knew the drop was because the card crashed out. I went back upstairs and sure enough, CGminer was crashed out (I SSH into my rigs from my main computer)....dam...