путаться в словах(unable or disinclined to speak freely, as from shyness: I'm normally very fluent, but if I'm tired, I lose my connection to words and I get tongue tied... I'll either forget how to say a word...4uzhoj)
быть не в состоянии выдавить из себя что-то внятное(be at a loss for words) напр., от волнения и т.п.: I get tongue tied and can't think of the words to say. Because of this problem I've been failing every single job interview.4uzhoj)