If you are running multiple GPUs and one or more lock up, that should not affect the others.

If it is not a lock up you can see higher CPU load on their python processes
http://forum.bitcoin.org/index.php?topic=30513.msg383127#msg383127


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...