Device hangup problems

Hi!
Im having problems with routerboards across the entire network these days,and cannot figure out what is the cause of hangup of devices.

Its a simple bridged network,so all the interfaces are bridged together in the rb’s.
There are no aditional configuration on the boards.
All working good until a power cut,or just until the power cord gets unpluged.

This happened on RB750GL,RB411 and RB433.

Lets take an example,everything is working good,and the power cuts for ie. 2minutes.
When the power comes back you can hear that the device beeps once,then 2 times (normal bootup),but this is where the problem comes,the ethernet port LEDs are flashing on both the rb and the switch,but i cannot reach the routerboard,nor over ip or mac, doesnt show up in neighbors,and no traffic is going through.

The only way to get it back is to unplug the network cable,bootup the device,and then plug the ethernet back,and everything works until the next powercut.
I have solved this with UPS unit at the site,but sometimes when the batteries get flat,i have to drive to the site and do the mentioned trick.

Same symphtoms on all board types 750GL,433,411.
Upgraded to latest ROS,upgraded the firmware,but the problem still persists.

Any help is apreciated.
Thank You!

Problem solved!
It was a faulty ether1 port used as uplink on 750 as well as on 433,they are all still working as ehter2 is used for uplink,we are not using ether1 ports anymore for data just for powering the device.
The RB411 has been replaced.