There are a lot of things that may be contributing to you getting errors. I personally like to overclock one thing at a time, so I would drop the multi as low as it will go for now and work on getting yourself stable with that FSB frequency.
Is your ram set to factory specs for voltage, and list the northbridge volts as well as fsb termination if you can. At over 400FSB you may need a termination voltage of 1.3-1.35 and the northbridge might need a bump as well. Just raise them slowly and one at a time, then test.
Once you get a stable system at the FSB frequency you want and the ram is running well, move on to overclocking the cpu.