I have a very similar system. Same MB,CPU, case. I'm OC to 4400 without a problem. Are you using the stock HS? I use a CoolerMaster Hyper212+. As to PSU I use an Antec Truepower II 750W. What video card are you using? I have 2 5850s in Crossfire. How many fans in your case? I added 2 200 mm coolermaster fans and used a Zalman 120 mm for the exhaust. Sounds like either a ram problem I use Gskill ddr3-1600 or PSU. Could be that your heatsink is not properly attached.