CPU Issue X6 1090t

My friends X6 1090t will some times turn to a 5 core and deactivate the 3rd core in the bios. Its set to auto and when it deactivates it goes to manual and its never just a random core its always the third. Would this b a sign of that core being bad? maybe try settings to manual with all enabled and see if it happens still? He current does not have an overclock, rarely enters the bios so it wouldnt b a accident. His motherboard is a Crosshair IV Formula and some of the programs run performance mode automatically changing things but dont think it'd deactivate a core.