Internet accessibility issues

Internet access has slowed to a halt on our network; we are running W2K Pro and IE6 on the workstations and W2K server on all servers. All are fully patched and updated. We access the internet through a Cisco 2600 series router and Pix 515e firewall. Once the router and firewall are restarted, the internet works fine for a minute or so and then freezes up. We are running the latest virus sig. files, and we have scanned all the servers and no signs of a virus. We have not made any changes recently except adding managed Dell switches, although even those devices within the hub still cannot access internet.

Thanks,
Dan