messages backing up - help

[Chuck Rock]

I am running FreeBSD 6.2 with MailScanner 4.61.7 and ClamAV 0.91.2 on a Dual
Xeon 3.0GHz with 2 Gig RAM server.

This box is just a pre-filter to do AV scan and the basic RBL and phishing
scan. No spamassasin filtering on this server.

I have been messing with the Max Children, Max Unscanned Messages Per Scan
and Max Unsafe Messages Per Scan to find the magic numbers to get mail to
prcess through this server smoothly.

Max Children = 30
Max Unscanned Messages Per Scan = 40
Max Unsafe Messages Per Scan = 40
Max Normal Queue Size = 5

Current queue has 5000 messages waiting to be scanned. 
----------------------------
smtp1(1600):[/usr/local/etc/MailScanner]-#ps auxww | grep MailS | wc -l
      17
smtp1(1601):[/usr/local/etc/MailScanner]-#w
12:18PM  up 30 days,  2:38, 2 users, load averages: 1.47, 1.35, 1.37

Sep 26 12:18:25 smtp1 MailScanner[83495]: Batch (40 messages) processed in
52.51 seconds
---------------------------
smtp1(1613):[/usr/local/etc/MailScanner]-#ps auxww | grep MailS | wc -l
      32
smtp1(1614):[/usr/local/etc/MailScanner]-#w
12:20PM  up 30 days,  2:40, 2 users, load averages: 0.53, 1.10, 1.27

Sep 26 12:20:40 smtp1 MailScanner[83502]: Batch (40 messages) processed in
182.66 seconds

I can't find it.

I grep the mail logs for the scan seconds and I find this.

Sep 26 12:00:49 smtp1 MailScanner[78986]: Batch (40 messages) processed in
221.82 seconds
Sep 26 12:01:07 smtp1 MailScanner[78940]: Batch (40 messages) processed in
239.80 seconds
Sep 26 12:01:09 smtp1 MailScanner[79037]: Batch (40 messages) processed in
237.63 seconds

Then: 

Sep 26 12:08:46 smtp1 MailScanner[79484]: Batch (40 messages) processed in
667.70 seconds
Sep 26 12:08:53 smtp1 MailScanner[79207]: Batch (40 messages) processed in
691.09 seconds
Sep 26 12:08:56 smtp1 MailScanner[79329]: Batch (40 messages) processed in
678.39 seconds

And this time will continue to climb. 

Earlier it was: 
Sep 26 11:52:53 smtp1 MailScanner[64885]: Batch (40 messages) processed in
1272.42 seconds
Sep 26 11:52:54 smtp1 MailScanner[65051]: Batch (40 messages) processed in
1291.63 seconds
Sep 26 11:53:36 smtp1 MailScanner[66205]: Batch (40 messages) processed in
1186.13 seconds

I restarted MailScanner and it goes back down for a short time.

I have another server similar specs, FreeBSD 6.1, 4 Gig RAM, Dual Xeon
2.8GHz same MailScanner and Clam version. It's time also climbs, but takes a
long time and the /var/spool/mqueue.in directory doesn't fill up with
messages waiting to scan. This server can handle about twice as much
throughput as the other. I also have the MailScanner.conf settings as above
set higher. This one has 20 children with 80 messages per scan. The time
rarely gets above 300 seconds in the mail logs.

How can I troubleshoot this? Obviously it's not going to be fixed by
tweaking those MailScanner settings above. I just don't have any idea why
the scan times creep up like that.

Any ideas?

Thanks,
Chuck