Freezing eMac

I've got an Emac running Panther that has started locking up for no apparent reason. It will be sitting idle and when I come back to it, there is no mouse cursor, the screen is black, computer is unresponsive. I can force a powerdown/reboot and then it is fine for a while.

Any ideas?

Thanks

I would run Disk Utility and Repair Permissions and Verify Disk.
Also, check the logs using Console. There is a crash log as well.
(applications/utilities)