Hi Conrad,
your problem sounds familiar. I guess it's the same bug which has been reported earlier by several moodlers in this thread:
http://moodle.org/mod/forum/discuss.php?d=51536
You can find it also in bug tracker: http://tracker.moodle.org/browse/MDL-6494. There you can find more information about this bug, as well as a partial solution made by Ari Tahvanainen.
- Jani -