hello,
I have a great trouble with MT, sometimes i cannot get any request from MT, no winbox, no telnet or ping. mb is Intel® Desktop Board DG31PR, ethernet is onboard Realtek* RTL8111-GR Gigabit Ethernet Controller?, but Mikrotik says (in driver list) RTL8169 Gigabit Ethernet. When i unplug and plug again RJ45 jack Mikrotik rises and works some time again and than I have that trouble again and again. RouterOS 3.1, Mikrotik uses for wi-fi bridge, switch Dlink 3526.

I would try another NIC (just for test)

Yes, use another Ethernet Card.

Yes, trouble was with ethernet card. Reconnected Mikrotik to gigabit switch, in gigabit mode adapter works properly wtf _).