Intermittent USB

I have an Apple keyboard and Microsoft mouse connected to a USB hub to my Power Mac G4 AGP, and the USB cuts out occasionally, and takes a restart to restore it. I've tried reconnecting the mouse and keyboard directly to the USB ports on the back of the computer, when they went out last, but I got no power to them. Could this be the motherboard at fault? If so, I've located an exact replacement board on eBay, that's real cheap, and happens to be from the same company I bought the Power Mac from. I can use the computer for hours, before the USB cuts out, forcing me to reset the computer by the button on the front, although sometimes I was able to get the USB back by disconnecting the USB hub cable at the back and refitting it, that only worked a couple times. This happens both in OS X Panther and OS 9. I've reformatted and reinstalled everything.