PowerPC :: G4 Screen Goes Blank And Sound Cease

I've got an 800Mhz iBook G4 (640MB RAM, latest update for Tiger 10.4) which has worked without a hitch for the last 2 years. Suddenly it has started going dead while in use - the screen goes completely blank (although the backlights are still on) and all sounds cease. It doesn't matter if the machine is idle, or playing World of Warcraft, or encoding a DVD in DVD Studio Pro. It is completely random. The only way to get out of it is to hold down the power button to force it to shut down. Powering up is also becoming difficult - 9 times out of 10 the machine boots but I get no startup sound. Instead the screen stays dark and the fan goes on full throttle, and nothing happens. Sometimes I can force it to shut down by holding down the power button, sometimes I need to take the battery out. When the machine does boot and I get back into OS X, sometimes it displays a message saying my clock and time/date are set before 2001 and I may have problems.

I understand this message comes up if you reset the power management unit, but on checking the time/date it is set correct. I have tried resetting the power management unit, but it doesn't make any difference. I have run the hardware test CD, and all items pass on the extended test. The iBook's battery is almost dead - this I know! But would that make any difference? With the time/date error that sometimes comes up on restart, it's almost like the internal battery that stores data like the time has run flat. Is this charged by the big battery? I haven't booted into open firmware, but I've read I can run the reset -nvram and reset-all commands from there (not sure what they do). It's not hardware, as the hardware test CD works. It doesn't seem to be a software error (although I haven't reinstalled OS X). Has my RAM gone bad or become unseated?