Do you have it patched to at least SR2 (SR3 is available and includes the earlier ones)? Reason I ask was that this was a known problem prior to P5 SR2 on certain machines (exact spec never determined) in which the P5 Renderer would fail, and the Interlok copy protection would force a reboot. I'll spare you the details on how/why. From SR2 the Interlock code is unlinked from P5, so if P5 fails it's not likely to trigger a reboot.