new cpu causes unwanted reboots

i have an aopen ak72 motherboard thats suppesed to take up to 1gig slot A athlon.
i currently run a 500mhz chip and its fine.
i bought a 900mhz chip and when i put it in it cause the comp. to reboot out of no where. ( niether chip is thunderbird) bios reads the 900mhz chip correctly with the correct core voltage but seem to be running cool. i have 384mg pc133 ram. WHATS THE FIX TO THIS PROB.?? LOL BESIDE JUST USING THE 500MHZ CHIP?