My 5yo eMac (1GHz G4 with 1GB of RAM) freezes intermittently. Sometimes the mouse & keyboard lock up, sometimes a semi-opaque grey 'curtain' covers the screen with a message window in the middle telling me to restart. Sometimes it restarts okay, sometimes it freezes on the Apple/cog-wheel screen, sometimes the grey curtain comes down again, sometimes the desktop loads okay but then lines of code scroll down the left side of the screen and it locks up again. The RAM is third-party stuff but it's been working fine for over 12 months. The freezing occurs under 10.4 and 10.3.