Other saw issues as well on the 360 with Ghosts. I could not get in to MP until I shut off the 360 then back on. My buddy's shut off the consoles, router and power cycled the ISP modem, then powered on the router and then did a factory reset on the 360s. NAT was open after that. Seems that this is a XBL or Ghost issue. If this is the only game doing this, then I presume your router settings are fine.