It is observed such on rb411 in different places on v5.2 all well
07:23:37 interface,info ether1 link down
07:23:39 interface,info ether1 link up (speed 100M, full duplex)
07:26:27 interface,info ether1 link down
07:26:29 interface,info ether1 link up (speed 100M, full duplex)
08:04:27 interface,info ether1 link down
08:04:28 interface,info ether1 link up (speed 100M, full duplex)
08:04:35 interface,info ether1 link down
08:04:37 interface,info ether1 link up (speed 100M, full duplex)
08:07:32 interface,info ether1 link down
08:07:34 interface,info ether1 link up (speed 100M, full duplex)
08:09:12 interface,info ether1 link down
08:09:14 interface,info ether1 link up (speed 100M, full duplex)
08:12:09 interface,info ether1 link down
08:12:11 interface,info ether1 link up (speed 100M, full duplex)
08:28:57 interface,info ether1 link down
08:28:58 interface,info ether1 link up (speed 100M, full duplex)
Note: v5.3 contains a problem that affects RB400 series device Ethernet ports which are not connected to the switch chip. The ports could lose link for a fraction of a second. A fix is scheduled within a few hours
wait v5.4?..
i have the same problem with RouterOS 5.4 
I use RB411AH
any idea ?
what is on the other end of the cable? maybe cable is bad, or other end of the link is acting up
i have RB1100AHX2 10 ethernet ports
interface,info ether1 link down
interface,info ether1 link up (speed 100M, full duplex)
i have connected here 411AH, 433AH RB711 all same problem. i have PPPOE server running on all these ports
and pppoe clients disconnect for this problem
i am running version 6.0rc14
check cable, poe and… that you have power enough trough poe.
in the other end is RB1100AHX2 and i have a pppoe server running in ether1 to ether10
all ten ethernets same problem
Try change ethernet cable