Interesting ... sorta port flapping. What is connected on that port?

I have same problem in some routerboards...;((

I had this issue with port 1 of my MikroTik Powerbox. Try lowering the MTU. That was the only resolution I found.

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why?

Check your Ethernet cable on ether4
If you have an Ethernet cable tester , then test it
If you have a crimping tool, cut off both ends and make new ends.
How long is the Ethernet cable ?
Make sure both sides are set for auto auto

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why?

Check your Ethernet cable on ether4
If you have an Ethernet cable tester , then test it
If you have a crimping tool, cut off both ends and make new ends.
How long is the Ethernet cable ?
Make sure both sides are set for auto auto

You are right, when ethernet goes down and up all time the problem may be:
1.- Bad connectors (make them again)
2.- Bad Cable (replace them)
3.- Distance of cable (max recommended for PoE is about 70-90 meters)
4.- Ethernet interface damaged on external/internal unit

Regards.

I had this problem before.
RB850G connected with two ADSL modems and one AP.
The whole network was not steady, ADSLs disconnect randomly, WiFi lost connection.
After checked the log, I found a lot of link-down and link-up, the interface will be suddenly dropped to 100M and then back to 1G but after some time it will go back to 100M.

I replaced all the cheapy thin flat LAN cables with a branded patch cords.
All problem solved.