I would step up the OC immediately to 1100 and then wait to make sure that the WUs processed at that speed turn to "Valid" on the WCG site (easiest to check by time returned). After that I would start stepping up small increments (say 50 at a whack) and follow the same pattern of checking the WCG site. I have not yet crashed a WU completely on my 7850 although I did push it far enough for a "driver reset" to occur. I just bumped the volts a hair and am currently running 1250 for about 6 hours. 4.2 on the CPU is good but these WUs do really benefit from higher OC there as a good deal of processing happens on the CPU at each start (depending on CPU and RAM anywhere from 12-42 seconds) and about 23 times longer at the each end of a WU. Keep in mind that these tasks actually have 2 WUs wrapped up inside so you'll see a pause at in the middle also (49.707%).