There could be a number of reasons for this, but more information would be needed for an accurate diagnosis.
There are some concerns that pop into my head instantly:
You said that you're taking the exam in September, but that these aberrant scores happen every tenth test or so when you practice. This leads to at least three questions:
1.) How often are you taking these tests?
2.) How much do you review the tests after you've taken them?
3.) How many tests have you taken?
The first two questions can be most important in diagnosing your problem. First off, if you're simply taking these tests back-to-back on a daily basis, you could just be experiencing fatigue by the time that 10th test comes around. If they're more spaced out, then what are you doing in between tests?
One thing you should be doing is reviewing any work that you do. Reviewing work is vital to understanding what went wrong on any particular question or test. You said that you retake the test next day and score back in your normal range. While that's great, do you understand what went wrong the day before? Do you remember what your frame of mind was? Was there one question that threw you off and set off a chain reaction? Do you really know what was tempting you away from the right answers? If you don't analyze this, your susceptible to making the same mistakes again.
Finally, that last question -- how many tests have you taken? And what tests are they? Are you currently using older exams or more recent exams? Plus, the September exam is still three months away! Are you worried that you might run out of practice materials by then?
Of course, a lot of this also depends on how long you've been studying, what you've used to study to bring you up to such a high score range, and a few other factors. If you share a little more information, perhaps we could help you out a little better.