A couple of questions about this statement:
"During filesystem backup when monitoring the network interface the speed is very intermittant. The backup concurency is 8. It is retreeving data from 5 different servers. Large and small files."
How many devices are being used? What is the total number of Objects being backed up (You can get this from the session statistics printed at the end of a backup)
My first impression is that your concurrency is not set high enough to keep your tape drive streaming. If you have enough objects being backed up, a concurrency value of 10-15 is probably realistic
However, there comes a time when Objects start finishing while others are still running. While we would like to have all objects finish at the same time, this is not realistic. So, much of the time, you will see what appears to be a slow down in the backup rate near the end of the backup, when there are not enough data streams going to the tape drive to keep it going without stopping and waiting for the tape buffer to fill