V-ray backup to MSDP hangs in periods

We are in the process of change to V-ray backup over SAN to MSDP on our VMWARE installation and see some hangs in the backups we cannot explain, as neither CPU or ram seems to be heavy utilized at the time on the media

Wondered if others had seen something similar

Systems:

Netbackup 7.5.0.5 on Windows 2008R2 Master & Media Servers, which have 96 GB ram & 2 x8 cores CPUs

VMWARE vsphere 5.1.0 update 1on ESX hosts and vsphere 5.1.0 on VI Center