Googling around, there are a heck of alot of replies for "XFX 5870 XXX", XFX HD-587A-ZND9 model number seems prevalent.
Lots of reinstalling of Win7 without joy, dropping in old cards making the systems work ok again. Seems to me that perhaps XFX have dropped the ball with this revision.
I've seen a good number of people dropping the clocks to a standard 5870, similar to what u've done, and they seem to work ok. Funny things happening before desktop initially opens up, grey screens, strange pixels - things u may associate with a bad oc.
Chances are XFX may have shipped a standard bios in these by mistake and it's as simple as the voltages being incorrect or being self-monitored wrongly. Feelings there and about are that they're behaving like standard 5870 cards with XXX stamped on them.
Personally, I drop a card in a known working system, and I've hauled arse with drivers etc to get it working correctly @ stock levels and fail - I'm immediately returning it the store I purchased it from - forget XFX rma, the purchase is too new. Just return it as not working correctly.
Some other manufacturer issued cards with the incorrect bioses in them not too long ago. I would wage decent cash on these things barely hitting the news tho as it bucks the recognizable trend.
Imo return the card asap for refund, or exchange it for a sponsored ASUS model. ASUS may be a number of things, but their gfxcards have impressed me quite a bit l8ly.