Slow Backup

Using Arcadia Backup Exec and an HP Dat 8 tape drive, I am backing up data from 4 servers. Each of them has between 1 and 1.5 GigaBytes of data, and takes 20 to 30 minutes – Except one which takes 30 minutes every night for a couple of weeks, thentakes over 5 hours for several days, then goes back to 30 minutes. This pattern keeps repeating. Does anyone have any idea what could cause this, and is there any way to determine if specific files or directories are creating a problem?