postmaster service seems to stop without reason

Hello!

Another problem just occurred: We've got a postgres database running
okay, we can access it an everything seems fine - most of the time. It
seems however, that the postmaster stops sometimes and restarts without
our notice - we had a look over the event-log and found several entries
"postmaster stopped" or "postmaster started" - but neither of us
restarted nor rebootet the server, no one stopped or restarted the
service.

This in itself wouldn't be much of a problem if no one would notice. It
seems however, that the postmaster sometimes doesn't delete it's .pid
file, thus refusing to start up again on the next occasion.

I have got a workaround in place which just deletes the .pid before
restarting the service should it fail, but this seems to be to be a
quick and dirty solution: I'd like to know why it stops in the first
place - and why it sometimes doesn't delete it's .pid when it does so.
Is it meant to behave like this?

Any ideas?

Thanks,

Markus