Try pulling the plug to the internet and connect to a lan only game. IF this works fine, then you're computer and the server are both sending packets thru the net and when the router recieves them back, the port forwarding takes over and the router is taking your packets that are supposed to come to the client and sending them to the server. I had this problem with hosting Serious Sam 2 over my network in it's early stages. Also if you get errors running ping 127.0.0.1 on the server or your gaming rig, that means that your tcp/ip stack is probably screwed up. You should get 0ms min, max, and avg response time w/ 0% packet loss. If it's anything other then that, there's a serious problem w/ ur tcp/ip on that machine.
