As you raise the FSB, the memory is also raised. So you may be at a point where the memory is causing stability problems, so you either want to set a divider, or lower the RAM speed (which ever it is that your mobo supports).
So say you have DDR2-800 RAM. In some BIOS', you can set it to run at DDR2-667, although since you overclocked it, the RAM will be near stock.