Yes this happens to me too sometimes, especially under situatuons where I have to answer questions under a pressure situation and many people are present (presentations etc). I think it's because my brain is so focussed on being fluent that my thought process gets overwhelmed and I start to utter gibberish. What helped a bit was consciously waiting with my answer for 10 seconds and using this time to put together my answer in my head before starting to talk.