Mac Freezes constantly!?

O have a Powermac g4 Sawtooth With AGP,runs 10.3.9 and 512 mb of memory, it seems to freeze randomly but mostly when i do eXpose or minimize something. i can leave it on all day and will not freeze but when i use it regularly it freezes like if its in the middle of music the music will restart every second, and my mouse freezes to it just stops everything mouse music and all and i have to press the power on for 5 secs to force it off. It is interrupting my work. maybe ram or hard drive the hard drive makes loud cracks when accessing alot of data. What would Cause This!?