I am basically looking for two words which are related to some degree. Ultimately the same word might even be the answer to both parts of this question. I split them up into two questions (see also Technical term for the loss of words in your mother-tongue when speaking a different language).
Anyhow, here's what I'm trying to give a name.
If you think about a random word – take any – and you say it repeatedly, over and over again, and think even more about it, then – sometimes – it may happen to you that the word sounds more unfamiliar to you, and you start wondering if this word exists at all. Then, if you use it in a specific phrase or sentence, you also start doubting if you can use this word in that context at all.
Is there a technical term for the loss of understanding of words or phrases which you actually not only know by heart but also are able to use off-the-cuff under normal circumstances?