SLP here, there are many factors as to why some people are fluent when drinking. One reason could be your are not "thinking" about what your are going to say as much. When tipsy ideas and words just come out even for regular people. For most people who stutter, as the cognitive complexity of the speaking task INCREASES, fluency DECREASES. For example, when I curse after stubbing my toe on the coffee table...no stuttering. The word "dammit" just comes out as a reaction. No cognitive complexity. When reading aloud many people who find it easier because they are reading what is already in front of them (anxiety may make it worse but that is not the topic right now). Trying to explain something to a peer is more cognitively complex and dysfluencies increase. Debating or defending an opinion or idea is even more complex resulting in more dysfluencies. Clear as mud? I would try to find some literature on this, however I am using the mobile app at the moment .