I agree with ananke. My mind often is speeding away. Most of my actions are fast, and around half of the time, my speech is as well. I am not a very garrilous person, but when I do talk, my words come flooding out. Like you, I mix up words. It can be bothering, but it isn't too big of a deal. At first, it was humorous, and now it is 'annoying.' Some things, you don't want to mix up. For example, I once mixed up Huck Finn in class, and since I don't swear, that was upsetting.
When something happens, and I share my thoughts with a friend, they often feel as if I think too fast. I will tell them three different thoughts that occurred in about one second. The mind thinks of much more, but it sometimes seems like a lot. Trying to speak slower, and contemplating what you plan to say, can help to prevent mix ups. I still occassionally mix up words as you do, but not as often.