Whenever I OC and run a stress test, my cpu will run at the speed i set it at.... it then randomly drops down to 852mhz for a few seconds and then goes back up to the normal.
Then check your Temps if it gets to hot it will throttle it down to 800 mhz or 852 in your case since it is overclocked. Use real temp or core temp to see how hot your CPU is getting. Are you overclocking on stock heatsink?