Recurring "error code -50/invalid argument" resolved by reboot, better solution sought

I'm using an old iBook G4 (PowerPC) running Tiger to serve iTunes to the house. For the past week or so, the mac boots fine, and then sometime during the day starts generating An unexpected error occurred (error code -50). errors when iTunes attempts to save its library file. Checking reveals that the boot drive (main drive, in the laptop) behaves as if it is write-protected - I get the same error code when I attempt to create a folder on the desktop, for instance. If I attempt to create files via the terminal, e.g. via ls > foo.txt I get an "invalid argument" error. I've set the iBook to run in Target Disk Mode and run Disk Utility to repair the disk (from a Macbook running Snow Leopard) and get "The volume appears to be ok". I don't have the option in Disk Utility to Repair Permissions in Target Disk Mode. When I ran Repair Permissions on the iBook itself, it came back with no errors, no permissions corrected, but did not in fact fix the problem. Rebooting always solve the problem - for the next 5-10 hours or so. Would Disk Warrior help, or some other software fix, or is it time to buy a new drive? (And, in any case, what's going wrong?)