I bumped into this a few times: Try
1) Run a cleaning Tape (a new one)
2) Reseating the Cables and SCSI Card and Tape Drive.
3) Check your SCSI Termination - Reseat that
4) Check the settings on your SCSI Card. (Are they optimally set for that tape drive?) (Sometimes you can slow down throughput to see if it clears up.
5) Select a different channel on your SCSI Card.
6) Try a different SCSI Card.
7) Try a different Tape Drive.
kevin