If a core is unstable on your hexa-core contact AMD today and let them know whats going on.
Since your underwarrenty and this is not your fault user end, they will gladly ship you a new one.
Ask them if they can send you one and you send it back to them today, some companys will do this others won't depending on circumstances.
-Quoted from Newegg.com AMD cpu page-
Manufacturer Limited Warranty period (parts): 3 years
Manufacturer Limited Warranty period (labor): 3 years
Manufacturer Contact Info
Manufacturer Product Page
Website: Global Provider of Innovative Graphics, Processors and Media Solutions | AMD
Support Phone: 1-408-749-3060
View other products from AMD
This item is covered by Newegg.com's CPU Replacement Only Return Policy.
Return for refund within: non-refundable
Return for replacement within: 30 days