IBM Type 2628 laptop shuts down.

Please, I have a PIII 750Mhz IBM Type 2628 Thinkpad 21M laptop running Windows XP pro. When I turn on the laptop, the CPU fan turns for a few minutes then goes off. While working, the laptop power goes off from between 1 minute to 12hours from the time I begin using it. I’ve changed the Hard disk and memory. I’ve also removed the modem, leaving only the basic components to load up the computer, but the problem persists. Can someone please help me out?
Thanks much.