Slow Backups with BackupExec 10d after Windows Server 2003 migration

After upgrading our servers to Windows Server 2003 Standard, backups using Backup Exec 10d have gone from about 1gb per minute to about 140mb per minute. We are using IBM ULT3581-TD2 tape drives. We upgraded BEX to service pack 4, and upgraded the tape device drivers to the latest, as well as upgraded the drives’ firmware. We have done everything Symantec tech support has asked us to do to no avail. Also, the backup times have been inconsistent per server. First out Domino server was slow & our File & Print server was fast. Now it’s the opposite. Any suggestions?