I have the same problem. My friend and I like to talk about books. I find my stutter is always at it's worst when I'm talking to her about a book I like. It's really annoying because if I'm just talking about neutral stuff my stutter isn't as bad. The main theory of why this happens is because these are "charged words". Meaning, words with lot's of emotions attached. Similar to why lots of people struggle to say their name. I don't really have any tips, I just try to slow down and word switch like crazy but it doesn't always work.