I'm not running crossfire, But i am running a 5870 and i can produce the same error as you have stated. As soon as the error hits the system will restart. I can open as many CPU-Z windows as i like without a hitch. My HT-Flood errors hit when i do any hard I/O task. I have 3 systems with this hardware and only one does this. I have not had the error since flashing to the newest bio's revison.
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