I have the exact same thing. I notice that it fluctuates depending on how tired I am, If I've eaten or my general mood. It is largely non existent when I'm with my girlfriend, because im comfortable with her. The block varies with other people. The one thing I do when I block is quickly form synonyms, or say what you want to say a different way. Then, I try and avoid synonyms and use difficult words with people I am more comfortable with and who I know are aware of my impediment. Couple this with reading aloud helps. Make sure when you read aloud you're doing it consistently, and really *emphasise* the words or sounds you would otherwise struggle with in a social setting. Moreover, when people make jokes like that don't pay too much mind to it. They are just unaware, as most people they encounter do not have speech impediments. Or you could explain it as simply a speech impediment and you struggle with certain words. It's easier than you think and most people are very understanding of it. Good luck friend, hope this helped.