have you done anything like update the operating system or install/remove any apps or their extensions?
also, what version of mac os are you using? 9.1, 9.2.x, OS X (10.0 or 10.1.x), ...?
you should run disk first aid to check your disk (both the firewire and your internal drive) for errors. If you have norton disk doctor or some other drive utility, running that to check for problems is a good idea.