One thing to check (we had this issue a lot in the past)
if a host running a VM runs out of host DVS ports it will disconnect the nic, it will not ever reconnect until you go in vCenter and check the box.
Host->Configuration->Networking->DVS->Properties:: Maximum number of ports designated on this host. (if the 256 default this is EASY to hit in a cloud).. we had to use 1024 to stop this issue for us.
Not sure if it is the same issue, but this took a while to find when we hit it the first time.