Tape backup speed - Erratic

Hi,

We have a 570 backing up a 3TB 4.7 Unicode database (save while active) to a 3592 tape drive. Some weeks the backup takes 12 hours. Other weeks the backup runs 24 hours or anywhere in between. We have not seen any appreciable difference in the system load at the time the backups are running. Has anyone else seen anything like this? It's really puzzling.

Thanks,

Craig