Practice scores go down with stress?

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I've taken about 15 practice tests, from the 10 Actual and 10 More Actual Preptests books, and my scores over time have risen slowly, peaked very high and then started falling (my most recent score yesterday was ten points lower than my peak score). 

I've been taking the tests under slight pressure circumstances (public library, timed, no break between sections).  I'm starting to get bored and tired of taking so many practice tests--should I work through my boredom in order to build endurance, or should I take a break from studying to refresh?

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My timed average is probably about 171.  I started off at 170 and then peaked at 179 twice after about 10 exams...but then dropped down to 177 and then 169.  I'm kind of worried that as I get into the more recent tests, my skill set is getting more challenged?

A week-long break does sound like a good idea.  I'll be traveling for the second half of May, so that'll be some time away from this stress.

Thanks!