This is the second time in the last week I've read about an AMD CPU core disappearing. One guy had a quad core that turned into a single core-- at least thats what the computer thought had happened. The recommendation was: "Clear CMOS/BIOS and or load failsafe config in the BIOS."
They fixed it and said this...
"Loaded default config in bios, then windows reinstalled ACPI and prompted me to reboot, whereupon I found all my cores once more. "
Perhaps something to consider. I'm just routing information I read, I have no idea how relevant it will be for you.