em(4) link flapping

[Eugene Grosbein]

Hi!

My Intel motherboard contains on-boad NIC:

em0: <Intel(R) PRO/1000 Network Connection, Version - 1.7.25> port
0xac00-0xac1f mem 0xfe9e0000-0xfe9fffff irq 10 at device 1.0 on pci2

It it connected to Allied Telesyn AT FS724i unmanaged switch,
media is autodetected correctly - 100baseTX full-duplex.

It worked nice for 3 months. Today morning switch and em0
start to loose/establish link every 5-10 seconds. They do it
with autosensed media and with manually set 100baseTX (both half-duplex
or full-duplex variants) now. I've forced it to 10mbit/half duplex,
it seems to be much stable.

This box has two more PCI ethernet cards, each of which is two-port em(4),
so I have em0 (that has problems) and em1, em2, em3 and em4
(that keep working OK).

I tried to replace a cable, to reset the switch, to reboot this 4.9-STABLE
box - nothing helps. What should I try next?

Eugene Grosbein