Hi Cactus,
Thanks for your idea. I don't think it is related to reboot, but it may be related to post-upgrade times or something else. It doesn't happen all the time, but when it does start happening it goes on for a couple of hours, and the emails come through with the 15min cron jobs.
For example, on 21-Jan-09 it happened at:
1:30 PM
1:45 PM
2:00 PM
2:15 PM
2:30 PM
2:45 PM
I've deleted most of the admin emails that have this warning, so I can't work out the timing pattern yet. I'll wait until it starts happening again and see if the timing gives some clues.
Cheers,
Rishi.