I was using a 4870 X2 card.
One of my slaves (or sometimes two) would freeze.

And only a hard reboot of the system would fix it once that happened.
(Could continue to play the others, but nothing else would start up after the freeze, and once I closed the other clients, had no mouse control).

Changed to a GTX275, which is the equivalent to a 1x verson of my card.

WoW plays about the same as before.
Aside from the freezing issue, having completely disappeared.

Not sure if it was the 1x/2x cards on one board.
Or if it was the ATI issue.
The ATI card was just fine in my Q6600 system, but didn't like the i7 system upgrade I guess.