G4 PowerMac Freezes in OS9 & OS10 Hardware Problem?

I could use someone's help. I've got a couple of G4's w/dual 1GHz processors that tend to freeze up during use. No set time, just whenever it feels the need. What could possibly cause it/them to freeze up. Same result under OS9 and OS-X. What gives. Both machines are stock except for a 5-port USB card in one. I read where someone thought it might be a video card problem on their machine. Suggestions?

Thanks much.

could be a number of things, Video Card I doubt

What version of OS X do they run, OS 9 freezes are quite common but not OS X

Have you added anything else lately like RAM

Also what firmware update are the machines running and did you install OS X from full retail install discs and not grey coloured model specific discs?