Hang on - my overclocking experiences just gave me another idea...
Is there an option to set the processor and bus clock speeds in the system bios (i.e. soft setup rather than jumpers on the m/board)? If so is there any chance there could be a mismatch between the bios settings and what your processor is capable of? Perhaps you forget to change the settings after a flash bios upgrade? Or perhaps someone has been trying to squeese a few extra MHz out of the system?
An incorrect clock speed or multiplier setting could cause the system to freeze.
Just a thought.