My son is the same. He's very good in speaking and can speak complete sentence since 2 years old. However, he sometimes keeps repeating words or even stuck in the middle of a sentence and takes a few seconds to continue. This situation come and go for a couple times already and each time last for 1 day to over a week. i think as zitaeric said, our babies have learnt so many words and beginning to speak longer and more complicated sentence, they need to stop and think sometimes. No need to worry, this situation wouldn't last long. But please do not blame them on this as it would discourage them from speaking and would make situation worse. Just encourage them to slowly speak/express their meaning and no need to hurry.