When you updated your router's firmware, did you reset to factory defaults before re-configuring the settings? Doing a factory reset after the firmware update will properly sync the new firmware with the router.
For unstable wireless connection, configuring the wireless Channels will help. Set the Channel to non-overlapping Channels like 1, 6 or 11 for 2.4GHz and/or refer to this link: http://kb.linksys.com/Linksys/ukp.aspx?vw=1&docid=cf6ed888ce144d06905af4987887fd1b_KBxxxx_EN_v1.xml&pid=80&respid=0&snid=1&dispid=0&cpage=search