Are you on iOS 6.0.1? Try upgrading to that if not. I'm pretty sure that iOS 6.0 and a specific issue with the iPhone 5 caused issues with WPA2, whereby the connection would slow to a crawl or not connect at all, if you haven't already tried that then it may be worth a shot. Personally I never suffered these issues but I have noticed others who have suffered this issue. If you have no luck with that then like others have suggested try updating your router firmware.