Wierless Modem > Router > Xbox360 and NATS?

Hi I hope I don't come off as a compleat moron but here we go. I am attempting to avoid spending $89 on the Microsoft Wireless USB thing. So I am on a cable modem (Motorola SBG900 SURFboard) and I pulled out my old LinkSys Router (BEFSR41 v2.1) and use it to get on Xbox Live. Well for a week now it's running well and I ain't having any problems and then tonight I get kicked from a game and can't join any games. I can connect to Live but it won't do anything I keep getting an error message saying my NAT is set to moderate. I checked my setting on both the modem and router and they have NAT Enable or Disable. I've tried them all... Any thoughts or ideas to get me pointed in the right path?