Can’t connect to certain web sites

We are running a 5.1 Netware file server as well as a dedicated Border Manager server. We have two specific web sites that we cannot connect to. It appears as though the name is resolved however the connection times out. Also, when trying to ‘Ping’ it times out. If we do an ‘nslookup’ it comes back with an ip address. Trying to connect to the ip address gives the same problem. The BM server is set up with a public NIC and a private NIC. Currently we are using the BM server as a proxy server and DHCP but no DNS, however we were having the same problem when we were using NAT at the BM server with no proxy.
One of the sites we are having problems with is cdw.com.
Any ideas on what could be causing this?