Thanks for the reply!
I think out of courtesy, i should reply with what we ended in doing.
We had a setup that was very similar to the above configuration. We were using Cisco 4507e chassis switches to pass the iSCSI and LAN traffic between the VNX 5300 and the UCS. After packet captures and logs from both the VNX, UCS and ESXi , we decided to do a full reload of the 4507E. This then stopped the logouts completely in one site and vastly reduced them in the other.
Taking that this was very proplematic, we decided away from iSCSI and went to fibre channel. We since have not had a problem.
Regards
Andrew