Failed Backups

I have not been able to get a backup on my Dell PowerEdge 2600 for over a month now. They have replaced my Mother Board, Tape Drive and SCSI cable. Still am unable to get a good backup, I have tried Windows Server 2000 and Veritas backupexe and it always fails. I have updated drivers and bios no luck. My backups were going well and nothing new was installed or downloaded at the time of my first failed backup. It seems to get to the end and then just fails. Any Ideas help ???

Thanks EP