Wired USB Ethernet Connection disconnecting sporadicly

first, I'm not a Linux Pro, so please forgive me if I write any nonsence :)

So I have quite an unusual constellation.

I'm running a Ubuntu 14.04 Virtual Machine on my Mac Pro using VMWare Fusion 6. Because I have to forward ports (for an Owncloud Installation) for a specific and unchanging MAC Adress on my router, I have connected a USB Ethernet Adapter and connected it to the VM. Now everything workes fine for many hours but then the Ethernet interface looses its link to the switch. The Network Manager tells me that the Cable has been unplugged. After setting networking to disabled and enabled again the network reconnects. in a few hours it will disconnect again.

I tried 2 Adapters (both ASIX AX88179), 2 different Switches and of course different cables. I am sure that this method (conecting a usb ethernet adapter to a vm) workes flawlessly on both a Mac OS X VM (never had any network problems in 2 years) and Windows VM (had been running perfect for about 6 months).

Does anyone have an idea what is going on here? I added some systemlog messages but from my perspective it lookes like the cable disconnected:

Jul  8 12:39:01 Carbon CRON[3882]: (root) CMD (  [ -x /usr/lib/php5/maxlifetime ] && [ -x /usr/lib/php5/sessionclean ] && [ -d /var/lib/php5 ] && /usr/lib/php5/sessionclean /var/lib/php5 $(/usr/lib/php5/maxlifetime))
Jul  8 12:45:34 Carbon kernel: [ 1989.296243] ax88179_178a 1-1:1.0 eth1: ax88179 - Link status is: 0
Jul  8 12:45:34 Carbon NetworkManager[914]: <info> (eth1): carrier now OFF (device state 100, deferring action for 4 seconds)
Jul  8 12:45:38 Carbon NetworkManager[914]: <info> (eth1): device state change: activated -> unavailable (reason 'carrier-changed') [100 20 40]
Jul  8 12:45:38 Carbon NetworkManager[914]: <info> (eth1): deactivating device (reason 'carrier-changed') [40]
Jul  8 12:45:38 Carbon kernel: [ 1993.414886] IPv6: ADDRCONF(NETDEV_UP): eth1: link is not ready
Jul  8 12:45:38 Carbon avahi-daemon[679]: Withdrawing address record for fe80::a2ce:XXXX:XXXX:XXXX on eth1.
Jul  8 12:45:38 Carbon avahi-daemon[679]: Leaving mDNS multicast group on interface eth1.IPv6 with address fe80::a2ce:XXXX:XXXX:XXXX.
Jul  8 12:45:38 Carbon avahi-daemon[679]: Interface eth1.IPv6 no longer relevant for mDNS.
Jul  8 12:45:38 Carbon avahi-daemon[679]: Withdrawing address record for 192.168.0.XX on eth1.
Jul  8 12:45:38 Carbon avahi-daemon[679]: Leaving mDNS multicast group on interface eth1.IPv4 with address 192.168.0.202.
Jul  8 12:45:38 Carbon avahi-daemon[679]: Interface eth1.IPv4 no longer relevant for mDNS.
Jul  8 12:45:38 Carbon dnsmasq[1447]: setting upstream servers from DBus
Jul  8 12:45:38 Carbon NetworkManager[914]: <warn> DNS: plugin dnsmasq update failed
Jul  8 12:45:38 Carbon NetworkManager[914]: <info> Removing DNS information from /sbin/resolvconf
Jul  8 12:12:59 Carbon whoopsie[1195]: online
Jul  8 12:45:38 Carbon whoopsie[1195]: offline
Jul  8 12:45:39 Carbon NetworkManager[914]: <info> NetworkManager state is now DISCONNECTED
Jul  8 12:45:39 Carbon dbus[606]: [system] Activating service name='org.freedesktop.nm_dispatcher' (using servicehelper)
Jul  8 12:45:39 Carbon dbus[606]: [system] Successfully activated service 'org.freedesktop.nm_dispatcher'
Jul  8 12:45:40 Carbon ntpd[2009]: Deleting interface #5 eth1, fe80::a2ce:XXXX:XXXX:XXXX#123, interface stats: received=0, sent=0, dropped=0, active_time=1957 secs
Jul  8 12:45:40 Carbon ntpd[2009]: Deleting interface #3 eth1, 192.168.0.XX#123, interface stats: received=151, sent=151, dropped=0, active_time=1957 secs
Jul  8 12:45:40 Carbon ntpd[2009]: 91.189.X.X interface 192.168.0.XX -> (none)
Jul  8 12:45:40 Carbon ntpd[2009]: 217.79.X.X interface 192.168.0.XX -> (none)
Jul  8 12:45:40 Carbon ntpd[2009]: 134.106.X.X interface 192.168.0.XX -> (none)
Jul  8 12:45:40 Carbon ntpd[2009]: 129.70.X.X interface 192.168.0.XX -> (none)
Jul  8 12:45:40 Carbon ntpd[2009]: 109.75.X.X interface 192.168.0.XX -> (none)
Jul  8 12:45:40 Carbon ntpd[2009]: peers refreshed

For what it's worth, we had the same problem on a MacPro with Windows 7 on Parallels, disconnecting from the wireless. No-one else had problems keeping a connection to that wireless. When we changed the wireless router it started working fine. Still not sure why but we have had no problems since. It could be time to try and swop the network switch and test.