It's called a block. It's fairly common. They just don't show it on TV where they have (fake) stuttering characters. I do that, too. They're the worst, because no one else knows what's going on. There aren't really different types of stuttering, the way there are different types of diabetes or whatever. Stuttering is really the symptom. Unfortunately, no one really knows the cause. Some difference in the speech center of the brain, probably. I've heard that the reason we can usually sing fluently is because that uses the creative path in the brain which is different from the communication path. Make sense I guess. I've heard that if you do a voice or an accent, that can also make you fluent, same thing. Can't really sing when you're ordering pizza though. Alas.