Make sure your PPC is adequately cooled. Especially if it is has been upclocked. I have an issue with mine that if I don't have a fan running on the heatsink, I start getting random errors and sometimes it wont even boot. Took a long time to figure this out, especially since for a while it seems fine.
Subtle first signs were when it actually failed to copy files correctly. The files would copy but the copies would be different. Wrong CRC and whatnot. I thought it was memory first, but adding a fan made it stable.
