no internet access

We had a Cisco router that was working up until we reconfigured the billion router (form the ISP) Suddenly all the nodes connected to the newly installed core switch through to the billion router (default gateway) cannot access the internet. I can ping the local DNS server and the default gateway and the troubleshooting diagnostics produces the following results; Windows cannot communicate with the devices or resources (primary DNS server). When i unplug the cables and directly connect to the router, there is internet access but the connection is lost sporadically. (My cabling runs from the billion router to the firewall then to the newly installed core switch)