odd problem w/ either motherboard or processor

ok so my comptuer has been crashing randomly, ive been
getting BSOD w/ errors such as:
Driver_irql_not_less_or_equal, Page fault in non paged
area, system logon process failed.

changed the memory, that didnt help.

i have a biostar motherboard w/ an XP2000, and when i set
the bus speed manually on the motherboard to 200MHz
instead of 266Mhz, thus making the processor speed about
1250Mhz, everything works fine, no crashes.

so what is going on? why is the system unstable at 266MHz
bus? could it be power?