Those of you who are reasonably fluent, are there certain people who are your kryptonite?

Those of you who are reasonably fluent, are there certain people who are your kryptonite? Ugh. I just got off a Skype chat with a woman I'm mentoring. I'm covert and reasonably fluent, but for some reason every time I chat with her, my stutter rears up: blocking, repetition, the whole shmear. I have no idea why. I don't feel nervous around her -- I'm good at what I do and very used to having this type of conversation. Frustrating, but kind of amusing too. Does this happen to anyone else?