When setting the modem to periodically reboot (Services -> Auto Reboot -> Periodic Reboot) on 'Sunday' the modem reboots every day of the week.
This I believe is because BusyBox does not honor the '7' as a Sunday as 7 is non-standard.
I got around this problem by logging into the console and editing the crontab with:
# crontab -e
And setting the weekday position from '7' to '0'.