I am having the same issue.
I *think* it's due to the Mobo's FSB of 1333MHz and our CPU already being rated at that.
Our CPU's FSB are already rated at that figure, (333x4=1332) and as soon as you increase you are running over the Mobo's FSB.
I *think* to OC with this Mobo you need a cpu that can increase its multiplier higher than the 7.5 (and as such, can run at a lower Frequency) than ours will run.
I would like somebody to confirm this for me, as it's been a long time since I've looked at OC.
g41m combo has a fsb wall of 339mhz, anything over ok restart , only p 45 mono has 400 mhz fsb, and it is not possible to change multiplier , it is only possible if cpu is unlocked like amd be series and fx series, where u can increase multiplier