i've the same issue with mac pro 6 core westmere!
my solution are switched off hyper thread and use only 2 cpu phisich!
my question are:
i'ts a cpu problem or software ?
osx or logic??
I would want to know what of thoughts...
if shortly apple release a fix and the bug disapparead good,otherwise I will resend my mac pro in apple store and and I make me to replace with a new one and working!!
obviously it is still in guarantee with apple care and it is right that one that it spends 4.000 € for a mac pro it has a working computer!