Mac FileMaker server does complete shutdown mysteriously - need to find logs as to why

We have an important FM 11 server running on an 8GB SSD iMac with 10.8. Occasionally it does a complete shutdown for no rhyme or reason. I'm trying understand why and am curious if there are logs somewhere to see what triggered the shutdown.

When I am logged in via VNC, I have on occasion gotten kicked out because the machine attempts to shutdown or enable fast user switching (or so VNC says).

Any help is greatly appreciated.