I have the same card and it works great with the latest beta ati tray tools, so I don't believe its a problem with the application. Mine reaches around 775mhz but seems unstable when going from load to idle so the card shouldn't have any block of any sorts per max o/c.

ATI Tool worked but did not give me any readings.

Wish I could give you a solution but I hope this helps through deduction.

Cheers,