I remember this annoying shit. You have a thought in your brain and the mechanics in your mouth just lock up for no reason. I cured that by saying intro phrases that were easy for me to say. For example, I had no problem pronouncing words that began with the letter "S." So...I would begin talking by starting off saying "so" like this sentence. "So...wait a minute, you didn't check the movie times for the new Nicols Refn Winding movie?" "So like do you know where the financial aid building is?" So yeah, I need like a 30 bag of weed." or I'd say "Hmm" or "Hey" because "H" wasn't hard for me. Whatever intro word you can find that makes sense as a seque into what you really want to say, use it. You may start to sound repetitive starting every word with the same intro word, but fuck it, whatever works.