I'm not familiar with that particular board but bad OC'ing can have a couple of causes and they're all related to the quality of the board components. Sometimes it's something very simple like a marginal heatsink on the NB or SB that causes overheating. More often, it's the power delivery system using marginal chips, which causes fluctuations in voltage for the various components. If I had to guess I'd say it's the latter in this case - and probably no cooling on the power chips (to the left of the CPU).
As a last-ditch effort you might try cooling the power chips better. What CPU heatsink do you use ...?