Making many mistakes in the last 20 questions of the exam

Hi All,

I have given the GMAT before and I got a 680 (Q49 V34), and I intend to improve that score to around 720 (especially on the Verbal).

I have been preparing for more than a month now and I gave a few practice exams from MGMAT and Kaplan. (planning to give the real GMAT in Sep)

Though I have been working on various aspects of Verbal including my timing, there is one common trend across all the exams that is worrying me.

Across all the four tests, I realized that I do a lot of questions wrong in the last 20 questions of Verbal. My accuracy is decently good in the first 20 questions, while I do a series of mistakes in the last 20 questions (especially the last 10). This is not because of the time problem, as my accuracy goes for a toss even with decent time in hand.

Typically I tend to do 2-4 mistakes on the first 20 questions, but I end up doing about 10 mistakes on the next 20 questions.

I would like to know what could be the possible reasons for this problem and how can I correct this problem?