a strange issue that PHY cannot detects eth link up signal ?

Hi, we develop a board based on sam9m10g45ek evaluation board, the eth0 should be exactly the same as the eval board, uses macb.c as driver. Same phy chip. (In short, we cloned macb ethernet port HW & SW from Atmel)

Now once a while, we see eth0 link LED is off after rebooting the system.
1. From linux prompt, type “reboot”
2. Board reboots
3. If everything is ok, we should see green and amber LED for the ether port
4. Once a while, none LED is on from the above steps
5. We change the macb driver to be loadable module, when this happens, we rmmod, then insmod, won’t help.
“ip link” still says link DOWN.
6. Unplug and plug in the Ethernet cable solves the problem most of the time. But this cannot be a solution.

Have you seen this problem before ?

Any suggestion appreciated !