Not an expert, but i was facing the same issue. Basically under high load my server was randomly shutting down. I have connected the server to a APC UPS. When i dug deeper into my UPS settings, i realised that i had set the system to shut down when only 10 mins of runtime was left. However upon tracking my UPS status, under high load, my UPS was giving me a runtime of 8 minutes (less than 10 minutes). I believe this is what was triggering the shutdown.
I disabled the APC daemon and this seemed to fix the issue.
If you are using a UPS, maybe this could be the problem. Let me know if you would like more details !