em(4) regression between r289975 and r300212

This is an amd64 laptop, Dell Latitude 3340.

I updated from:
11.0-CURRENT #1 r298975: Tue May  3 15:18:03 BST 2016 /usr/obj/usr/src/sys/GENERIC  amd64

to:
11.0-CURRENT #2 r300212: Fri May 20 09:50:53 BST 2016 /usr/obj/usr/src/sys/GENERIC  amd64

Now em(4) stops working 2-5 min after boot,
or, perhaps, after 2-5 min of network traffic.

The interface is shown as up:

# ifconfig -a
em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=4019b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM,TSO4,VLAN_HWTSO>
        ether 20:47:47:01:62:6e
        inet 10.70.14.139 netmask 0xffffff00 broadcast 10.70.14.255 
        nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
        media: Ethernet autoselect (1000baseT <full-duplex>)
        status: active

and there's nothing obvious in dmesg or /var/log/messages,
but the network is not working at all, e.g. can't ping
/etc/resolv.conf hosts.

Is anybody else seeing this?

Thanks

Anton