amavisd-new exits unexpectedly

[IMAP List Administration]

Hello,

I run an OpenBSD mail server using postfix, amavisd-new, spamassassin, and
clamav. Amavisd is configured as a before-queue content filter.

I'm using the versions that come packaged with OpenBSD 5.1, namely:

> amavisd-new-2.7.0p1 interface between mailer MTA and content checkers
> clamav-0.97.3p3     virus scanner
> p5-Mail-SpamAssassin-3.3.2p3 mailfilter to identify and mark spam
> postfix-2.9.20120102-sasl2 fast, secure sendmail replacement

The other day I woke up to discover that amavisd had simply exited and postfix
was rejecting all mail to my server.  The last thing amavisd logged was:

> Sep  6 04:00:04 bsd amavis[10488]: Valid PID file (younger than sys uptime 3
> 3:34:00)
> Sep  6 04:00:04 bsd amavis[24627]: Net::Server: 2012/09/06-04:00:04 Server
> closing!

after that the rejections started.  Apparently amavisd restarts completely or
partially every day and I see the same message generated.  The difference is
that after the "Server closing!" message I see the usual amavisd startup
messages.  For example:

> Sep  5 04:00:04 bsd amavis[10885]: Valid PID file (younger than sys uptime 2
> 3:34:00)
> Sep  5 04:00:04 bsd amavis[17350]: Net::Server: 2012/09/05-04:00:04 Server
> closing!
> Sep  5 04:00:12 bsd amavis[7760]: starting. /usr/local/sbin/amavisd at
> dna.y42.org amavisd-new-2.7.0 (20110701), Unicode
>  aware
> Sep  5 04:00:12 bsd amavis[7760]: perl=5.012002, user=, EUID: 539 (539); 
> group=, EGID: 539 539 (539 539)
> Sep  5 04:00:13 bsd amavis[7760]: SpamControl: init_pre_chroot on SpamAssassin
> done
> Sep  5 04:00:13 bsd amavis[24627]: Net::Server: Process Backgrounded
> Sep  5 04:00:14 bsd amavis[24627]: Net::Server: 2012/09/05-04:00:14 Amavis
> (type Net::Server::PreForkSimple) starting! p
> id(24627)
> Sep  5 04:00:14 bsd amavis[24627]: Net::Server: Using default listen value of 128
> Sep  5 04:00:14 bsd amavis[24627]: Net::Server: Binding to UNIX socket file
> /var/amavisd/amavisd.sock using SOCK_STREAM

I did not twiddle with any software between Sept 5th and 6th.

Does someone have some idea why might be causing this, and how I might fix it?

cheers,

Rob