Yes I feel like that all the time. I started stuttering since I was 13 and I remember being a fluent speaker before that. I was a smooth talker, popular amongst my peers etc..but I don't know what the **** happened, I started stuttering all of a sudden. I am a covert stutterer. No one knows that I stutter except for my parents, my GF and a few close friends. They're all cool about it thankfully. Like you described, sometimes I think , "This is bullcrap. I can speak what I want, when I want." Then I go on ahead and have a fluent conversation for about a minute or two. Then I revert back to my old habit. It sucks. Maybe I need more practice. I don't know.