I have the exact same thing happen to me all the time. I will be talking, knowing what I want to say but somehow one word changes, making absolutely no sense.
This will also happen when I am typing.
I also run into the problem where I cannot remember a word and it drives me nuts. If I can remember a similar word I will look it up in the thesaurus. If I can remember the first letter, I will repeat its sound and can sometimes come up with it.
I find it happens more often when I am really tired or having problems with my depression.