I literally stayed up all night till about 5AM trying to get this exact card to hash over 550. The bios I flashed it with was one that changed made afterburner see it as a lightnining b to get a hash rate of 650 stable. It did voltage lock the card though.
I'll be home later today and will uploaded the bios I used since I don't remember the one I got from techpowerup.
I'm having this same issue with this same (rev 1) card. Can you please tell me what bios you used? I can't find one that seems to work.