Bonzo was most definitely drunk an uncountable number of times during shows. With that much drinking comes levels of drunkeness, and Bonzo was probably at least "drunk" at every show they ever played. Don't believe me? Read the October 2005 issue of Mojo Magazine. Robert, Jimmy, and Jonesy all said the same thing.
Read here on how the drums were recorded on Led Zeppelin III: http://recforums.prosoundweb.com/index.php/t/3883/0
12th post down, 5th paragraph.