It could be that your CPU isn't stable at that speed so the cores are failing. Have you checked stability with intelburn test or prime95? I personally test with both of those and even then I still use HWINFO64 to run a log file during my game play so I can keep track as to what each of my cores are doing once I've finished playing. This way I can look at the data collected and see how each core went up or down throughout the duration.