Yea, that is a pity man, hope you can rma the card.
I experienced like symptoms on a powercolor hd4870.
It is rma now, the reseller tested the card and confirmed it was broken.
Mine would freeze at stock memory speeds; both in windows and linux.
I bought a sapphire to replace it and that is fine.
Now i am only waiting for the reseller to send the other card back.