Random Freezes On a Cube

[jdench]

I have an upgraded cube, that has been running fine for over a year without any hardware changes. Just recently it has started to totally lock up. Nothing works, the clock stops, the mouse pointer refuses to move, nothing. I have to hold the power button down to restart. I have just upgraded from 10.3.9 (which it froze occasionally) to 10.4.2. Now it freezes more frequently. Sometimes the grey screen telling me I need to turn my mac off comes up, but normally it just locks. I have tried zapping the PRAM, clearing the cache folder but it doesn't make any difference. The only thing that is strange is that, under system profiler, the internal modem no longer shows up, but I can use it to dial out with. Please help me!