analyzing freebsd core dumps

[Mister Olli]

hi list...

I have a freebsd maschine running for more 6 months without any
problems.
the machine's only service is to be an openvpn gateway for a hand of
users.

2 weeks ago the first problems started. the openvpn exited with signal
11 and 4 and core dumps were written.

the same happend yesterday with the postfix/cleanup process, and the
suddenly the machine rebooted without any further log messages.

what is the best way to troubleshoot the cause of this problem?

greetz
olli