Technical term for the temporary loss of understanding a word 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?
Sounds like what you're describing is "semantic satiation". Wikipedia explains:


  The explanation for the phenomenon was that verbal repetition repeatedly aroused a specific neural pattern in the cortex which corresponds to the meaning of the word. Rapid repetition causes both the peripheral sensorimotor activity and the central neural activation to fire repeatedly, which is known to cause reactive inhibition, hence a reduction in the intensity of the activity with each repetition. 


English.SE also has some good answers related to usage of this term, rather than the neurological/psychological mechanics of the phenomenon itself.