[Exim] /var/spool/exim/db/retry.lockfile problems

I keep seeing messages in my mainlog:
Failed to get write lock for /var/spool/exim/db/retry.lockfile: timed out

if I do ls -ld /var/spool/exim/db:
drwxrwx--T   2 mm       exim         512 Mar 11 10:34 /var/spool/exim/db
and ls -l /var/spool/exim/db/
-rw-r-----   1 mm       exim           0 Mar 11 10:34 retry.dir
-rw-r-----   1 mm       exim           0 Mar 11 10:34 retry.lockfile
-rw-r-----   1 mm       exim        1024 Mar 11 10:37 retry.pag
-rw-r-----   1 mm       exim           0 Mar 11 10:34 wait-smtp.dir
-rw-r-----   1 mm       exim           0 Mar 11 10:34 wait-smtp.lockfile
-rw-r-----   1 mm       exim        1024 Mar 11 10:37 wait-smtp.pag

I've looked through the exim docs and the archives, but I can't seem to see any problems getting a write file lock for retry.lockfile.
This problem does not occur when the system is not under heavy load.
The load quickly goes from 0.19 to in excess of 4.0 when this happens

Is this some kind of race condition?

I am currently queuing mail, any help would be very greatly appreciated,

Alex