I've had a long term problem part time problem with vSphere 5.X whereas the occasional vm will lose network connectivity during a vMotion. It's a very intermittent problem, and there is no easy way to differentiate because vm's can have the same os and network adapter on the same host, but one will migrate fine and the other will lose network connectivity. I've become so hyper aware of the issue that I start pinging production vm's before I vmotion so I'm aware the second they go offline. A simple network adapter toggle (off, then back on) will usually kick the adapter back on.
I'm running vCSA 6 and I'm in the process of upgrading my ESXi 5.1 hosts, so I have a mixed 5.1/6.0 environment. I have had this problem ever since I created this environment at version 5.0. The one thing I have not yet seen is losing network connectivity when migrating to an ESXi 6 host. I don't know if that means anything, or if it's just because I still have a majority of 5.1 hosts and just haven't seen it yet, but I still wanted to mention it. I also run standard switches and have ample ports on each.
Anyone else experience this and know what might be the cause? I may open a ticket with VMware for the heck of it, but though I'd see what the community thought first.