I am not sure about the Xorg explanation, but let me try:
Xorg is attached to a one GPU, the only one that works with a screen when you boot your rig.
When that GPU lockes up, Xorg is lost...
Thanks, will investigate. I thought that Xorg lockup will make all the gpus not to work as the ATI driver is dependent on its hooks to X for its function.
I will keep an eye on that... I still have remote access when the lockup occurs so I will have a look if the other cards mine, I have only checked one of them and that one seemed to be stuck - phoenix still running and responding to new blocks but not getting results.