Lately it seems I have had to restart spamfilter in order for it to send through emails from the database that have been determined not to be spam.

I can't point my finger towards anything. Has anyone else experienced this?

I had a similar problem, it turned out to be my own fault though, but it might get you looking in the right direction.

I'm using SQL Server as DB backend and due to some enhancements (read:faulty triggers) I made on the database, the "delete expired messages" command from spamfilter always blocked other processes on the DB causing the "send tagged emails" command to timeout...

Your problem might not be the same, but nevertheless, I would check the Spamfilter log for timeout messages and the database for blocking issues.

Cheers, Mike