English word that means the use of out of place uncommon words

^{[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.)

I nominate erudition spikes.

Visualize a chart with the book's erudition level on the Y axis, time or position on the X axis. These sudden intrusions of sesquipedalian loquacity will appear as spikes on the graph.

More specifically,

the word is "Grandiloquent", if the writer has a tendency to use grand words, instead of common ones;

or

"Magniloquent", if the writer has a tendency to use long/large words, instead of short/small ones

Circumlocutious/circumlocutory are the words you are looking for.

M-w.com defines circumlocution (of which the above words are adjectival forms,) as :

1 : the use of an unnecessarily large number of words to express an idea 2 : evasion in speech