in our office we have a new windows 8.1 notebook. a user connects to a wifi network. it is working. after 5, 10 or 30 minutes a notebook disconnects from the wifi network, and the other laptops ( with w7) cannot connect to the wifi too. i changed the router. 2x. the problem still exist. if the notebook is powered off, the wifi is working well. if the computers cannot connect to the wifi, the wired connections still working well.

could you give me some idee, where should i start to find a solution?

thx, regards,