English word that means out of place uncommon word

^{[As requested, posting this as an answer instead of a comment]}

There's lexiphanic, which is using pretentious wording or language, but it doesn't have the sense of intermittence you wanted.

(I found the word by plugging "using long words" into a reverse dictionary. The first two results, sesquipedalian and sesquipedality, are also good, but they don't necessarily have a negative connotation like lexiphanic.)