Yes, it used to happen to me, too. My speech therapist advice: just stay silent until you feel that you are able to say what you want to say. It will seem like ages to you, so at first it may be difficult, but it's better understood and you focus on the real work you have to do, saying the word: unnecessary "um"s or "uh"s just delay a little bit the difficult word, but give you no real advantage. You will face the same challenge just one "uhm" later!