An update: If you are going to be in town between now and the 27th of September, there will be the Altstadtfest going on. You are certain to be able to get a good Frankische bier (and some good food as well!)
http://altstadtfest-nuernberg.de/
(Nürnberg seems to be having a fest going on every weekend. The Herbstvolksfest just ended and now there is the Altstadtfest going on.)