Does your system boot while running the processor at stock?
Do you have the CPU and PCI bus's set to Async (ie, PCI bus will not raise when CPU bus is raised)?
What brand power supply do you have?
Do you have Cool N Quiet Disabled?
Since you are running your CPU at 255, did you remember to drop your HTT multiplier down to 3x/maybe 4x (you might want to try running at 280 w/ cpu multi at 10x so you can set your HTT multi at 3x and not be underclocking it)?
may also have an answer for you, but we'll need the answer to the above questions anyways to help find out what's going on