are you using v8 Update 2 already or a previous version? At first sight, and looking at the size of the backup file (it's a single backup job handling all the VMs, correctly?) seems to me like you are hitting the repository cache too much and this has to flush data from time to time, especially after the job is running for some time. Out of curiosity also, how much RAM is installed in the windows repository server?
EMEA Cloud Architect @ Veeam Software
Veeam VMCE #1