Random Crashes and Kernal Panics

Hi,

I have a dual 2.3GHz Power Mac G5 (Early 2005) which has been working fine up until a few days ago:

I was typing something in Microsoft Word (2008) and the mac froze. I tried to boot it up again but it hasn't worked properly since, when you try booting it up it does one of 4 things:

-> Boots normally (but freezes again after ~30 minutes-1 hour)

-> Plays the startup chime but nothing else (no video or hard drive activity)

-> Boots and plays the startup sound but freezes on the grey apple screen (just before the spinning thing appears)

-> Powers on but doesn't play the startup sound.

I tried troubleshooting the RAM but didn't have much luck. When the Mac is running it shows the proper Dual 2.3GHz PowerPC G5 CPU and 2.75GB of RAM (4x 512MB sticks, 2x 256MB sticks and 2x 128MB ones) on the 'About this Mac' screen. I've also tried resetting PMU etc. but there doesn't seem to be any correlation between what I do and what the Mac does, it just does whatever it wants.

Yesterday I booted it up again using 2GB (2x 1GB sticks) of RAM that I took from my PC (and know to be working) and it recognised the RAM and worked for about an hour before giving me a Kernel panic reading:

System Failure: cpu=1 code=0000000A (Lock timeout)

Latest crash info for cpu 1:

Exception state (sv=0x3de97280)

I have the whole kernel panic photographed but I don't know how much is important (I can type out the rest if needed).

I realise it's an old Mac but it still works great (or at least up until now) and performs fine for what I need it for so I'd be grateful for any help, thanks :)