you forgot the wireless issue where ddwrt have decided to hard line stick to rfc specs meaning many users cannot get full speed N performance, and the few firmware builds where the ethernet ports completely stopped working O_o.
I read reports from others of various features not working in random 15x and 16x builds so reverted to the latest reccomended version from the main dev which is 14929. For what its worth tho I did use a newer firmware with the vmng300 before I downgraded and the ping graph had the same patterns. Your graph also has the patterns and you use a asus router.