I know this is a bit of an old post, and I'm not sure if it was ever resolved -- but I was experiencing a similar issue and figured I would add what fixed it for me in case it helps any one.
I was getting randomly disconnected when under moderate-heavy load (eg. svMotion or deploying from template) from my vSphere 5.1 host to my FreeNAS 8.3.1-RELEASE server. Every time it disconnected, it always involved iSCSI; NFS -> NFS worked, but NFS -> iSCSI, iSCSI -> iSCSI, etc would always break. The NAS box would lose network connectivity (thus causing vCenter/ESXi to choke and cancel the running task), with the only (temporary) fix being to restart the network interface on the NAS.
After much trial-and-error, and many hours of searching, I discovered that flow control on the NAS interface was the culprit. Once I disabled it, I was able to deploy templates or svMotion machines without fail and at peak sustained network speeds of ~122 MB/s over commodity NICs & switch.
I should note that I don't have jumbo frames enabled.
Hope this helps.
Server NICs, for reference:
FreeNAS = nForce MCP55 integrated
ESXi = Realtek 8168 integrated
Switch = TP-Link TL-SG1008D