The precise meaning of the word Berserk is not entirely certain yet. The image we get from them from sagas are diverse and confusing. Drugs might be an option, mental disabilities another and there are several more theories.
As for the Vikings, I think Top Gun is right. Being dead drunk is not a good idea in the middle of a battle. You might feel invinvible, but in reality you will be slow as a slug and weak as a child. It is unlikely they drank a lot.
Of course afterwards it is a different story...
(btw, moved to Medieval History)