You are not the first to have had CPU's just suddenly die like that. In most cases where this happened, it was always the CPU that died, as a replacement worked fine, but no one determined why it died. Whether the motherboard killed it or the CPU decided to commit sudoku.
The only way to troubleshoot this is to unfortunately try a new CPU. I'm sorry.
Contact Intel for a RMA? Just do NOT tell them that you used XMP. Period.