Hay all.
As i see this problem daily in my job, i would say to you that the DHCP (USUALLY ON WINDOWS) is having problems.
Best thing to is play with the DHCP settings within the administrator tools in the control panel. This usually seems to work.
Another thing to try which can bit a bit O.T.T is do a hard rest on your your router;Modem because DHCP resides in these aswell
Best Of LUCK