Not connecting after restart

Seems like the server is running pretty smooth, except for one thing.
Sometimes when I restart, the server just doesn't seem to want to be connected to. cannot ping the IP, cannot access from explorer on client PCs, and cannot connect to the web GUI.

Oddly, another restart or two and it returns to normal, and so far once its work it stays working, until a bad luck reboot makes it undetectable once more.

I haven't ruled out a network issue yet, but if anyone has any other ideas what may be causing this please let me know.
Thanks.

have a look in the dmesg log and see what the NICs are doing on boot...