One Core Hotter and slower than others

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http://superuser.com – I overclocked my Intel Core i5 3570K to 4.2GHz at 1.1V. It ran stably overnight on Prime95, but, while I was monitoring the core temps, they stuck within 45-55C with core #3 was always 3 or 4 degrees higher than all the other cores. Then after 11:50 Hours I stopped the workers and saw that all cores completed around 810, except core 3 which did 707 tests. Do I have a defective CPU? Background Processes? Or should I higher (or lower) slightly the voltage? (HowTos)