BackupExec fails difference in file data

Using Veritas Backup Exec 8.6 with HP SureStore DAT 24i (brand new), Adaptec AHA-2930CU card (also new), all new tapes. NT4 sp6 server.

Most of the time, verify after backup fails with “difference encountered in file data” in log file. One to several file names listed, depending on the day.

I’ve changed the max burst rate down from 20 to 10 on the controller – no effect. What else should I try?