By default, if your computer is using DHCP it is configured to hold onto the IP address for 8 days. Afterward it would usually sent a unicast requesting that it keeps it's original IP address, most likely if your internal IP address keep changing you might have to check the router or better yet if you think it might be a DHCP issue, then disable it and assign a static address for your computers. It's also good to check your NIC cards and cable just to verify that they are working well.