eNewsletter Cron not waiting correct time between messages

As mentioned in another thread with somebody experiencing a similar problem.

The CRON job seems to send out 150 emails per hour - as set
But it isn't waiting the required time (30 seconds) between each message, instead it seems to ignore this figure.

The result is that 150 rush out in about a minute then it waits another hour and sends the next 150 in a minute.