Computer sporadically freezing

So I have an e8400 on a evga 650i ultra motherboard. I've overclocked to 4.25 ghz on 1.38V. The ram is at 855 mhz on 2.1V. The northbridge is 1.5V, and the southbridge at 1.4V. The computer does not fail memtest, or prime95 after running for hours. The cores idle at 35 and under full load go to 59.

So in that sense the overclock is stable. But my computer freezes sporadically on mundane tasks like surfing.

Has anybody seen anything like this? Is it just a poor north bridge? I've already maxed out the voltage on my ram, cpu fsb, and north bridge, so adding more voltage to anything other than the cpu core is not an option.

I'm going to lower the clock and see what happens, but would love any advice.