overclocking the p4b-lx

i have a sony vaio case with stock cooling. i have a p4 1.5 on an asus p4b-lx micro-atx motherboard. i have it clocked at 1.68 but it is not to stable there. is this being caused by lack of cooling or voltage or what? im not new to pc building and repair, but new to overclocking.

any help is good. oh, and what is best for testing cpu speed?

thanks