Are you using an Adsl, or fiber optic modem?
I presume with either you are connecting to it via the Ethernet provided.
If it is not a router, but just a modem.
Then the cause could be software firewall related.
If you are using Windows fire wall or security center at the same time as another software fire wall at the same time disable at least one of them.
As two can cause a conflict and the loss you say you are having.
If you are having a problem, with Adsl, or a fiber optic connection, the fault or drop out in service may be due to the quality or state of the line or signal.
Through quality of the line, or signal power.
So you should contact your ISP and have them check the line.
When ever you install a new motherboard you must install a fresh version of windows.
You cannot use the same installed version of windows if the two motherboards differ in board chip sets.
You will have errors.
Because all of the old drivers from the old board and the settings in windows will lead to a conflict.
And often a freeze, blue screen or part function of hardware.