This is partly one of the ways my social anxiety manifests itself - thoughts or concepts I can write fluently*, suddenly become hard to express verbally. I find that the next word vaporizes en-route from brain to vocal cords, or that a sentence constructed on the fly, ends up going nowhere.Worse still, once you start to become self-conscious of doing this, you almost certainly condemn yourself to doing it, exactly when you least want to.

The weird thing is that whenever I hear or read something that's grammatically, or verbally suspect, my brain latches on to it like a terrier - I think that paying too much constant attention to grammatical structure & intended meaning can sometimes result in your losing the plot yourself...

*I don't usually have the same trouble with writing, though I may spend longer than most, looking for the right word, or way to put something down - since I typically have the luxury of time when writing, I don't get stressed about it.

The only way around it I've found so far, is to somehow extract myself from of the situation & gather my thoughts before trying again.