Why does my G4 keep freezing?

Using Tiger and Panther my g4 tower has continued to freeze more and more frequently. The freezing occurs at random moments, independent of anything I'm doing and affects the whole computer, not just the mouse and keyboard...everything freezes! I have tried clean re-installing the OS a few times and it still comes back. I also regularly use the program Applejack in single user mode, which seemed to reduce the freezing but now it has come back with equal and ever-increasing voracity! The computer was purchased in 2002. Surely there has to be a way to cure this. Does anyone know what it is, other than a really annoying disease!

There is about 50GB free space left on the 80GB hard drive.

General info

Machine model - PowerMac3.6
CPU type - PowerPC G4(2.1)
Number of CPUs - 2
L2 Cache(per CPU) - 256KB
L3 Cache (per CPU) - 1MB
Memory - 512 MB
Bus peed - 167Mhz
Boot ROM version - 4.4.8f2

I guess this info in the list specifies the variant.
I'm not sure how to test how much RAM is being used at a particular moment but it's not like it's freezing during heavy processes - it's only being used for word processing at the mo.

Hope you can help furtherly. Any suggestions more than welcome.


Ross