i210 fails loopback (offline) testing

I have two custom ARM i.MX6 I210 systems, A and B.

System A works fine, establishes link and passes the loopback/offline test.

System B brings up the ethernet interface but never establishes link (100 or 1Gbit), and the loopback/offline test fails:

ethtool --test eth1

The test result is FAIL

The test extra info:

Register test  (offline)         0

Eeprom test    (offline)         0

Interrupt test (offline)         0

Loopback test  (offline)         13

Link test   (on/offline)         1

System A has an I210 (WGI210AT) connected over PCIe to the i.MX6. It uses the built-in phy and runs Kernel 3.0.35 and IGB driver 5.0.6. It has INVM image 'Copper NoAPM v0.6'.

System B has an I210 (WGI210AT) connected over PCIe to the i.MX6. It uses the built-in phy and runs Kernel 3.14.79 and IGB driver 5.0.5-k. It has INVM image 'Copper NoAPM v0.6'

This failure occurs with or without magnetics connected.

Could anyone suggest what might cause this type of failure? I believe that PCI communications with the I210 is ok given that the driver can communicate with the part. I have tested multiple instances of system B and all fail in the same way, so I don't believe the I210 is faulty.

Thanks

Simon