Google came up with loads of stuff but i'm still none the wiser.

This was taken from a link about the Xeon

Systems with Echo TPR Disable (R/W) bit (bit [23] of IA32_MISC_ENABLE register) set to '0' (default), where xTPR messages are being transmitted on the system bus to the processor, may experience a system hang during voltage transitions caused by the power management events.

Implication:
This may cause a system hang during voltage transitions of power
management events.

Workaround:
It is possible for the BIOS to contain a workaround for this erratum. The BIOS workaround disables the Echo TPR updates on affected steppings.