You dont clear the BIOS, unless your board is one of the ones that requires manually resetting the jumper if it doesnt boot.
There are two major factors that can attribute to an overclock becoming unstable, which can be dealt with in the BIOS.
First off, once you overclock and it's no longer stable, dont start cranking up the voltage. Theres a high possibility that your RAM is overclocked too high, so either set a higher divider or lower the memory speed, or you can increase the RAM voltage/raise timings, your choice.
After you've done that, then you should start increasing the voltage... slowly.