My q8300 wont overclock to 3.0ghz?

so i have been overclocking my q8300 with an asus p5g41c-m lz motherboard. i have 4gb ddr2. if u dont know the q8300 starts at 2.5ghz. i have been adjusting my fsb that has a multiplier of 7.5. i can get my fpb to go to 399, which makes the cpu run at 2.99Ghz. now if i take the fsb to 400, then the overclock dosnt work and i can even boot. i have tried adjusting my voltiges, i have them at a good setting, but changing them dosnt help me take the fsb to 400.