If it continues to do it even after you reinstall, I would start suspecting something hardware-related, and I would start with the RAM first. Have you added extra RAM to the iBook, or, perhaps, did it come with some RAM installed? If so, try removing it and leaving the iBook with just the built-in memory and then testing the iBook. RAM-related problems are many with Panther.
Also, when you do reinstall, I would suggest installing on a cleanly formatted drive, and running Software Update immediately after you get done installing until no more updates show anymore. Leave all of them checked and just let SU do it's thing. When you're all the way back up to fully-installed clean system, repair permissions. Then, make a good effort to install all your applications one by one in the default locations (don't change the default install locations of your applications). When they're all installed, then run a permissions repair again (not in between each app install; that's pointless).
I would highly suggest not copying over old preference or application-settings files -- it doesn't take but a minute to set your app preferences, anyway, and it guarantees a cleanly written, non-corrupt preference file.
I've used this method many times and it always provides me with a good, clean, stable installation of software that keeps running error-free. It pays to be careful with an OS install and do it right other than hurry through it and miss a step and end up with an unstable application or system.
Good luck and let us know how it turns out!
