Why am I getting the question mark folder icon repeatly

I have an iBook 800MHz 1.2GHz running Leopard that I received as compensation for some work I did. At first the computer would only boot to the question mark icon folder, so I assumed that it needed a new OS, a new hard drive or both. But after installing a new OS, reseating the hard drive cable, trying a different hard drive, and then replacing the hard drive cable the computer still only boot to the question mark folder. It wasn’t until I removed the hard drive from the iBook and installed it in an external enclosure then I erased it, reformatted it, repair the hard drive, and then re-install Leopard that I was able to get it to boot up. However, after about an hour of testing and running apps the computer will starts to periodically freeze up and will soon need to be rebooted. The more the computer is used the more problematic this becomes until it will again show the "I can longer find the start-up drive icon". At this point it is no longer visible in Disk Utilities or Target Mode. The only way out is to remove the hard drive and start the process all over again. I would like to know if anyone here has experienced this or have any idea about what is going on? My final conclusion is that the logic board I bad.

The iBook has a 512 ram chip installed.

Thanks Mayer.