Wow! What an arrogant society we live in. The correct phrase is, "all of the sudden," it means suddenly (in the sudden moment of time). It is an adverbial phrase. If you have a phrase, "all of a sudden" it must mean (in a sudden moment of time). When speaking of a specific moment of time, does your usage of English allow you to say, "in a moment of time?" Mine doesn't because that phrase is used to refer to how long something takes, or how long something took. It does, however, allow me to say, "in the moment of time" referring to a particular moment. So, we say, "all of the sudden," not "all of a sudden." I have often suspected that, "all of a sudden" was a somewhat ignorant attempt to repeat the phrase, "all of the sudden." After reading these posts, I can see it has gained approval and acceptance and is viable in today's English. Fortunately, language, unlike nature, actually does evolve. Unlike the fossil records, we can look at the linguistic records and find evidence of how and sometimes why it changed over time. We live in a backward society. We think language should be treated as exact, but that it is okay to believe in a theory driven science (but, of course, that is part of another battle).