Arduino Ethernet with PoE stability issues

Hello,

I have several Arduino Ethernet boards with PoE.
They all seems to be prone to below described defect. Defect IS NOT present if I power these arduinos with 5V or 12V power supplies.

DEFECT:
They work for few hours flawlessly, and after certain time if i try to power relay board from 5V supply boards die, they arent anymore responsive to ethernet commands. There are no memory problems because there is no issues whatsoever if I power these boards from 5v supplies. I use certified 802.3af switch (netgear fs108p). Problem isnt random, they always work for certain time, even powering that relay board, and boards always fail after 3 or more hours when I try to power relay board (they seems to be working until i set digital high pin for the realy module).
I use these relay modules: http://www.microbot.it/en/product/75/Relay-Module.html

Thanks for assitance,