I/O Device error during Backup

I have a Dell PowerEdge 4300 with Seagate DAT drive for backup, on a SCSI raid controller. O/S is WinNT4 for Small Business Server, Backup Exec v7.3 SBS. I get a device I/O error in my backups, but it is not consistent. It can error one day, but work for three or error for three days and work for one. Has anyone experienced this problem an if so what was the problem and the solution? Thank you.

John