spam acl condition: cannot parse spamd output

I'm getting a few of these in the exim_paniclog

I cought the last time it happened as I was trying to SSH in, and found it took ages, and then found that spamd was running at over 90% and stayed there for almost a minute before returning to normal. I checked the logs for activity under the user account that was running spamd at such a high cpu quotient, and found the "spam acl condition" error. It's the only thing which shows up in the panic log:

cat /var/log/exim_paniclog
2009-09-21 09:59:15 1Mpeji-0004Oz-6e spam acl condition: cannot parse spamd output
2009-09-21 10:18:36 1Mpf2Z-0005q4-3d spam acl condition: cannot parse spamd output
2009-09-21 17:01:25 1MplKI-0001SH-4y spam acl condition: cannot parse spamd output
2009-09-22 12:10:23 1Mq3GB-0003WH-N4 spam acl condition: cannot parse spamd output
2009-09-22 12:10:36 1Mq3GH-0003WK-Fb spam acl condition: cannot parse spamd output
2009-09-22 14:05:58 1Mq52w-0003uP-Df spam acl condition: cannot parse spamd output
2009-09-22 14:08:24 1Mq54s-00040y-KE spam acl condition: cannot parse spamd output
2009-09-22 15:56:48 1Mq6n9-0004P8-PD spam acl condition: cannot parse spamd output
2009-09-22 15:58:12 1Mq6oh-0004XB-PS spam acl condition: cannot parse spamd output
2009-09-23 00:33:32 1MqEpe-0005Np-O4 spam acl condition: cannot parse spamd output
2009-09-23 14:08:33 1MqRZq-0006oW-L3 spam acl condition: cannot parse spamd output

I'm not sure why this is happening... server is not really heavily loaded, has 2Gb RAM and a quad core, with only about 150 mailboxes over 50 or so websites.

Interestingly, I have looked at the transaction for each of these messages, and in each case the size of the message was bigger than 7Mb, ranging up to 30Mb.... but I have already been told by Cpanel themselves that such large messages should not even be scanned by SA, so why is exim complaining it can't parse the output from SA?