I've been having this issue for a while now, and it's really driving me crazy. I have a small network with two windows 7 machines and one windows xp machine. All three machines are set up to accept remote connections through the router, and they're all set up the same way. The router forwards like so
IPAddress:3390 -> Windows 7
IPAddress:3391 -> Windows 7
IPAddress:3392 -> Windows XP
The weird thing is that the windows xp machine always works, and the windows 7 machines have had sporadic issues. Right now all three machines are on and connected to the internet, but I can only RDP into the windows xp machine.