Hi, I find some similarities to my story. I talked normally until at my 15. At the high school in the class, suddenly I wasn't able to read out loud some word at all, so everyone stared at me, waiting, but I couldn't say it. This was my first total block. Really unpleasant experience. After then I felt like my process of producing speech was somehow changed in my brain. Strange subtle feeling. Then I had similar experiences as you are describing, this was a very mild stutter though. This also started only with some words. About 2 years ago, my stuttering gotten much worse, but I still consider it as mild. I have blocks much often, some repetition. But if I can give you an advice, don't hide it, don't be covert stutterer as I was for 7 years. This constant word replacing is frustrating. Also, I developed avoidance behavior after my stutter gotten worse, because I was still trying to hide it. This is the way to hell. Btw this post is very inspirational https://www.reddit.com/r/Stutter/comments/5z1lhr/stuttering_my_personal_approach/