Slow Backup from local server

we have a LTO4 drive hooked into a server running backup exec 11d using SAS.  performance was great and still is from other networked servers but the performance when backing up files from the local server has recently halved.  we were getting around 2000MB/sec but this has now fallen to 1000MB/sec.  I cant see any issues with the server and the only thing that has changed is that the server has had all the Windows Updates installed and rebooted.

Any ideas would be grateful

thanks

Nigel