I googled this and find some people also have this issue.
http://www.symantec.com/connect/forums/iscsi-targe...
http://www.experts-exchange.com/Microsoft/Applicat...
Seems exactly same as my issue:
I have a windows 2008 R2 server and running hyper-v and connect to a iSCSI Initiator on that server.
When I run the backup from another win 2008 r2 (BE2012 and the latest version) backing up the files into the share folder (the iSCSI Initiator drive).
At the beginning the speed seems fine (up to 2000MB/min), when each bkf file completed (for example, if the file is 4GB and reaches 4GB/8GB/12GB etc.) and writting into the disk, the speed will get slower and slower and finally it will fail.
It seems the iSCSI Initiator disk dropped from the network, I can't access it from windows explorer, but after a while the iSCSI Initiator drive will be up again.
I don't know if there is solution for this issue, also don't know if this is a issue with Microsoft or Backup exec.
Thanks
Jason