I setup a home network for someone and it was fine with 2 machines. Their modem stopped working and Comcast gave them a new one. At the same time, I have since added a few more machines and a Sorenson VRS into the mix they are having problems when more than 2 machines are on the network at the same time. The Sorenson is a video relay service which allows deaf people to communicate via video. http://www.sorensonvrs.com/
The Sorenson equipment came with the Dlink Router and specified that I add 192.168.0.125 into the DMZ. I tried setting this up without the Dlink Router but then the Sorenson wasn't working. All the machines have been scanned for virus and have the latest security patches. Both routers have the latest firmware.
Here is the setup. The cable modem is plugged into the Dlink Router. Off the Dlink Router I have plugged in the Sorenson VRS, a Dell Laptop, and the Linksys Router. Off the Linksys Router, I have plugged in 2 desktops and there are 2 laptops that connect to this router via a wireless connection.
Cable modem -> Dlink Router -> Sorenson VRS
-> Dell Laptop
-> Linksys -> Dell Desktop
-> Dell Desktop
wireless (Dell Laptop)