motherboard ethernet port failing

[D. Hugh Redelmeier]

I have a Gigabit GA-P43T-ES3G motherboard (socket 775) with an
on-board RTL8111/8168B ethernet controller.

The day before yesterday, under Ubuntu 10.04 LTS, the network
connection went away.  I poked around with Network Manager but had no
luck getting it to work.

I had two different USB to ethernet adapters that I tried but Ubuntu
10.04 didn't recognize them.

The on-board adapter started working again.  I took that opportunity to
upgrade Ubuntu from 10.04 to 12.04, and that worked.  The first USB to 
ethernet adapter that I tried worked under 12.04.

Then eth0 stopped working again.

I'm pretty sure that it is the adapter or connector because I did a lot of 
experiments:

- switched ethernet cables

- switched ports on the ethernet switch

- was able to get other computers to work with the same cable and
  switch port

- (eventually) was able to get this same computer to work with a USB
  ethernet adapter and the same cable and same switch port

There were several reboots during this time.

When it is not working, ifconfig shows that the interface is UP.  RX
packets can be non-zero but TX packets are 0.  I never see any error
count.  Here's an example:
eth0      Link encap:Ethernet  HWaddr 1c:6f:65:a0:67:27  
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:29 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:3750 (3.7 KB)  TX bytes:0 (0.0 B)
          Interrupt:45 Base address:0xc000 
Notice that no address has been assigned (normally done by my DHCP server).

'mii-tool -v eth0' says:
eth0: no link
  product info: vendor 00:07:32, model 17 rev 2
  basic mode:   autonegotiation enabled
  basic status: no link
  capabilities: 1000baseT-HD 1000baseT-FD 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD
  advertising:  1000baseT-FD 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD flow-control
  link partner: 1000baseT-HD 1000baseT-FD 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD flow-control

'sudo mii-tool -w eth0' says:
20:12:28 eth0: no link
20:12:38 eth0: negotiated 100baseTx-FD flow-control, link ok
20:12:39 eth0: no link
20:12:46 eth0: negotiated 10baseT-FD flow-control, link ok
20:12:47 eth0: negotiated 10baseT-FD flow-control, link ok
20:13:11 eth0: no link
20:15:11 eth0: negotiated 10baseT-FD flow-control, link ok
20:15:12 eth0: negotiated 10baseT-FD flow-control, link ok
20:15:51 eth0: no link
20:17:57 eth0: negotiated 10baseT-FD flow-control, link ok
20:17:58 eth0: negotiated 10baseT-FD flow-control, link ok
20:18:30 eth0: no link


>From my observations, I think that it is a hardware problem on the
motherboard.  It could just be the connector.

Does anyone have an idea of what's likely going on and whether it
might be something easily fixed?

Any suggestions for experiments?

(I don't want to break the machine: it is a "production" MythTV box.
The USB interface solution is OK.  The box doesn't have any available
PCI or PCIe slots.)
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