NTBackup Slow

Hi
I use NT backup on an XP Pro SP2 client with a HP DAT72 tape Drive using USB
connection to the client to back up a share on a SBS 2003 Server. I do a
full backup each day approx 32gig. About 3 days a week the backup works fine
taking about 2 - 3 hours. However 1-2 days a week the selecting the files
takes longer and when the backup finally starts, it estimates that it will
take 1 - 2 days to complete! I cancel the backup take out the tape, switch
off the tape drive, count to 10 and try again. It may take 3-4 retries but
eventually it will work. It is if the tape drive is only connecting as USB
0.5 rather than USB 2.0
Any suggestions?
Thanks in Advance