I have the same problem in Ubuntu 14.04.3 too...here's my /etc/network/interfaces.
# Localhost
auto lo
iface lo inet loopback
##########################
# WAN - connected to MODEM
##########################
# We use allow-hotplug here as otherwise get boot hang if modem down
auto eth0
allow-hotplug eth0
iface eth0 inet dhcp
I have the same problem in Ubuntu 14.04.3 too...here's my /etc/network/ interfaces.
# Localhost
auto lo
iface lo inet loopback
##### ####### ####### ####### ####### ####### #######
# WAN - connected to MODEM
#####
# We use allow-hotplug here as otherwise get boot hang if modem down
auto eth0
allow-hotplug eth0
iface eth0 inet dhcp
The problem comes and goes.