My ibook G4 keeps restarting on its own!!

hi

"I have had this laptop for 4 yrs now, and its never really failed me. I am running panther on it OSX 10.3.9. The battery is shot and has been for a while so i removed it and run my ibook only on electricity while plugged in. It seems like this problem improved a bit after I ran computer without the battery in it. Now it has started again, it can restart at any time. I have reset PMU and PRAM, and run disk utility as well as cocktail, and the problem still occurs. I called mac support as well but they had nothing useful to say, although they did not think the battery being shot or removed from the computer would cause this problem, I have not tried installing a new battery yet waiting to be able to afford it. Many times this problem happens after i try to wake the computer from sleep mode. HELP!!! any suggestions? I got to figure this out asap. Thanks."

thanks for the reply....no I dont have any third party hardware plugged in or installed, I have an external La Cie HD but its only plugged in when needed. I have had this ibook for 4 yrs now so the Hardware test cd is not available, I dont have any of the CDs that came with it when i bought back then. Do you think updating my OS to tiger and reformatting the whole computer could help? I am on Panther now.
The only thing is that I cant tell if it s a HW or Software problem so reformatting and updating OS might not help at all.....!