We have seen this case in a number of our Proliant servers since last year and have been fixing it as we see them. You may be interesting in this one too.
Advisory: (Revision) Integrated Lights-Out 2 (iLO 2) Firmware Version 1.81 (Or Earlier) And iLO 2 Management Controller Driver Version 220.127.116.11 (Or Earlier) – ProLiant Server May Unexpectedly Reboot And Display Event ID 57 Error Messages
Update: Tests in our environment has shown that despite the firmware and driver update from HP, we continue to see unexpected reboots. HP has agreed that this is an issue and will revert to us.