I'd try to flash with AMIBIOS flash utility, instead of F6. There's an option in that utility called "program all blocks". Just maybe.
http://www.ami.com/support/bios.cfm
You could as well e-mail Asrock about the issue, maybe they send you some new bios. With my older Gigabyte K8NF-9 I would contact Gigabyte support and they would send me beta bioses.
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