Here's my issue. I had to replace my motherboard recent (bad capacitor). I received the replacement and installed it (same as the previous one). Every time I try to boot XP or Vista (on different drives) the system restarts. I tracked the issue back to the QX6850. If I only have cores 0 and 1 enabled everything works. If I enable any combination with cores 2 or 3 the system restarts at OS loading.
My list of troubleshooting steps is long but I'm not including them to pervent ruling out fix that I might have overlooked.