system 'lock-up' and IO stalling

I have a mid 2009 macbook, running 10.6.8
Recently the system has been 'freezing' at random.First, the application I am using will 'beachball'. At this point I can cmd-tab into any other application, but attempting to interact with it will cause it to also 'beach-ball'. This includes the force-quit dialog.
This behaiviour is similar to the system 'stalling' while waiting for a sleeping external drive to wake up. The difference is that there is no external drive connected, and the only way out is to force the power off.

The interesting thing is that when this all happens, all disk activity appears to stop, and the internal hard disk eventually 'spins down'.uring

Does anyone out there have any idea why this is occuring?
I can rule out:
- a failing drive (I am experiencing no IO performance degradation or data corruption, and SMART reports everthing as a-OK)
- running out of memory as it has even happened when RAM use is very low (also have over 40GB free disk space)
- Overheating (temps do not appear related)

I am really at a loss as to why this is happening, Some possibilities I have thought of are a bad driver, faulty 'sudden motion' sensor, or some sort of kernel failure-mode which does not cause a 'kernel panic' screen.

The annoying thing about it is its random nature, might happen when using a lot of system resources, or just a little, might be after days of uptime, or a few minutes. Even happened once when I opened up terminal just after a cold boot.