Very common. There are even names for the things you mention: world substitution and time pressure. Most stutterers can speak completely fluent when alone or to animals. For this reason I see stuttering as a primarily psychological issue that manifests in a physical way. (Although some in here will disagree with that) What can you do about it? A lot of therapy program teach you to stop avoiding feared speaking situations, stop substituting words, become an overt stutterer as opposed to a covert stutterer. Basically you need to become comfortable with stuttering before you can become a better communicator. The idea here is that you can not work on something that you are spending all your energy resisting and trying to hide. A two week intensive program really helped me a lot with my stuttering. (AIS in NYC) There's a lot of good information out there. Gunnars Nieder's book is one of my favorites.