This may because your O.C. settings are to high or to low (voltages). As a system ages these settings may need to be adjusted for stability. In any case you should start back at the defaults and reset your O.C. How did you stress test the system; what settings did you use and did you do incremental adjustments with stressing to get to your final setting. Also if you just took someones settings for your own, be aware all samples of a given cpu vary. It is always best to arrive at your system settings with incremental steps and testing.