problems in server 2008 R2

I have a network with windows server 2008 R2 and Windows 7 on client side
Now the problem is that whenever i make a new domain it runs correctly up to 3 or 4 weeks but after this the systems on the client side pings the server with : : 1
Reply does not come from the server ip that is 192.168.1.2
Help me plz if u can