Damnit, I just spent a day and a half trying to remember the word "paradigm". For some reason there are certain words that I know well and love using, but whenever I think of the perfect place to use them I get a mental block and can't remember them. Often I find one word blocked by another; even though I knew it wasn't the word I was after I couldn't stop thinking "zeitgeist".
Another one is "nepotism", but I've been getting better with that lately.
So do you ever have this problem? Is it mostly certain words that do it to you?