spamass-milter stops working under heavy load

[address@hidden]

Our two sendmail/spamassmilter-0.2.0-3 relays are working fine when idling. But 
under more heavy load (20K/day) the process stops working after 1 to 2 hours.
Then we got only this messages in the logs:

May  9 07:06:23 rdh-relay sendmail[23731]: i4972NC4023731:
mail.mydomain.com [1.3.9.9] did not issue MAIL/EXPN/VRFY/ETRN
during connection to MTA
May  9 07:06:26 rdh-relay sendmail[23733]: i4972QC4023733: Milter
(spamassassin): timeout before data read
May  9 07:06:26 rdh-relay sendmail[23733]: i4972QC4023733: Milter
(spamassassin): to error state
May  9 07:06:26 rdh-relay sendmail[23733]: i4972QC4023733: Milter
(spamassassin): init failed to open
May  9 07:06:26 rdh-relay sendmail[23733]: i4972QC4023733: Milter
(spamassassin): to error state

We have scrubbed to external Linux Sendmail specialists and both ran out of 
ideas. 
They both confirmed our configuration is fine (all the relay prevention stuff, 
white/blacklists, DNS MX record checking, spamlist checking, ClamAV check et 
cetera).
hardware: HP Proliant DL360 (single 2,8Ghz Xeon, 1GB RAM, 2x 72GB U320 SCSI 
discs)
software:      
      -Suse 9.0 professional
      -sendmail-8.12.10-7
      -spamassassin-2.63/perl-spamassassin-2.63
      -perl-5.8.1-81
      -amavisd-new-20030616p5-23
      -bind dns 9.2.2
      -clamav-0.70-1
      -spamass-milter-0.2.0-3

Any idea?
  Regards
    Bernd