First you will be able to see the NIC use another vmnic in esxtop -> n (Sure you are already aware of this). If you want to find this action in the logs, take to vmkernel log. First find what the DV port ID your VM is using, use esxcfg-vswitch -l
Once you have this take to the vmkernel log and look for entries like this.
NetPort: 3101: resuming traffic on DV port 124
This should match up with the DV port ID above where you believe the flapping to be occurring. I just tested this today and it matched up. If you wonder what the terminology for the resuming traffic on DV port refers to, its as the failover happens you verrry briefly lose connection, this is it re-establishing.