Multiple issues. AirPort Extreme v5.7 with Linksys VOIP router problems.

Ok, so I have a cable modem in our office, a Linksys VOIP phone router, and two of the older Airport Extremes model: A1034. The Cable modem is hooked up with an ethernet cable to our VOIP phone router. The VOIP router has an Ethernet and an internet port, and two phone ports. The modem is plugged into the internet slot on the VOIP router, then the ethernet port goes to the Airport Extreme. I have an Airport Extreme in my room and setup it up as a relay station. Having it hooked up like this, the Wifi works, and when I plug anything in into the base station in my room via ethernet cable, it works on my MacPro perfectly fine, but when I plug ethernet into my Xbox 360 it works for about 10 minutes, I get lagged out of Xbox Live and then I can immediately sign back in. but it does this over and over. Now, the other problem is that I have a wifi card in my MacPro, and it says it can find the networks, but they never work. I either get, "cannot find the proxy server" or "no internet connection". This was working fine about a week ago. EVERYTHING was working fine about a week ago, I didn't change any of the settings and I don't know what happened. I know this might not be the right place to post this but I need help because this has just became a big problem.