[djbaxter]

Because of the way vBulletin handles cron jobs, the extact times that any cron job runs will vary (since it requires a page load to trigger the cron job).

Check how it's scheduled currently in "Scheduled Tasks". I have mine set to run as follows:

Day of the Month *
Hour *
Minute 37

i.e., every hour, every day of the month, at HH:37. I also have the number of emails sent out at a time set low (3), although for an initial run through the database I had it set a bit higher (I think 10).

That seems to work well for me.