instability & lock-up problems - HELP!

[Sydes]

I'm having really bad trouble with my setup where I can barely use Pro-Tools for around 5-10 minutes before it crashes or locks-up. The playback of my composition frequently stops with an error stating that I must increase the hardware buffer size. I've played around with all the available settings and never found PT to be happy.

When I try to 'save' my composition when playback is active, Pro-Tools will immediately lock up.
Also if select a new plugin into a track slot when playback is present, the same happens, a major lock-up that is difficult to get out of. When the system locks up, I get a loud buffer looping sound (like a snare being hit 100 times a second) and this repeats until I turn off the Digi rack or force the application to quit. Just to clarify, if I stop my composition, save it and the play again, it's usually ok, same goes for adding new instruments.

After PT has crashed, I 'sometimes' cannot re-boot and continue where I left off as I will be faced with an error saying the clock cannot read from the hardware. If I'm lucky enough to continue where I left off, the OS will force a shutdown with the 'power button splash' after around 4 or 5 crashes.

I'm hoping there is a simple explaination for my troubles. I used to have things working smoothly but I haven't touched my gear in 6 months. Now that I've come back to using PT I'm suffering with these issues.

Pro-Tools 7.3.1 cs4 running on the latest updates for Mac OSX Tiger.
4 x 3Ghz Mac Pro (mk I) with 10GB RAM and 1.5TB free HD storage.
Digi 002 Rack

Help!! Any advice would be appreciated.