[exim] Failed to get write lock

Hi,

I am getting multiple errors in my exim log.

Failed to get write lock for /var/spool/exim/db/retry.lockfile: timed out

My exim configuration is fairly simple. Exim is run with the flags -bd
-odq so that it queues all incoming mail. I then run a cron job every
2mins to process the queue.
This is a new installation running FreeBSD 7.1 on a 4 core system with
exim 4.69_3. I have another almost identical system that is not
producing these errors.
Greylisting, dns failures and other defers seam to cause the error to
start. This increases the load on the system to a point where it almost
unusable though if left alone it does recover.
Is there anything I can do to alleviate this problem. Googling suggests
that many people have seen the same thing but I have yet to find a solution.

Many thanks for any suggestions.

David