Three attempts: 640-660-680. 4th attempt? And could I improve further?

I took GMAT 3 times:
19 June 2015: 640 (Q50, V26);
21 August 2015: 660 (Q51, V27);
13 December 2015: 680 (Q50, V31).

As you can see, my problem is Verbal. I took the first GMAT Prep test after reading through Manhattan Strategy Guides and Bible (for CR), and got a V31.
After that, I used Verbal Online from e-GMAT, but got V30 on the 2nd GMAT Prep test (V30-V36 on Manhattan tests), and then a disastrous score for the 1st real test: V26.

For the 2nd attempt, I used Verbal Live course (from e-GMAT too), but still V31 and V27 on GMAT Exam Pack 1 tests, and then V27 on the 2nd real test.

After the 2nd attempt, I spent about 3 months on preparing application, and focused on GMAT for last 30 days. This time, I do not use e-GMAT anymore but focused on official questions and Questions Pack 1. For CR, I was back to Bible. Also, I had a habit of reading 2-3 articles on Economist everyday. The accuracy for Questions Pack 1 mini-tests (which are a mix of easy, medium, and hard questions (more medium ones) ) was quite good from 80%-90% (Yes, I know the accuracy and percentile do not quite correlate to each other). The most improvements were, I notice, in RC and CR. Some SC questions still seemed to be confusing to me. And I attribute a V31 in 3rd attempt to the improvements in RC and CR.

I think my most deadly problem is my stamina and probably the anxiety. Yes I did a lot of CAT tests (in real condition) before each real exam, but I don't know why in real tests, I kind of lost all of my stamina as I reached Verbal section. I hardly understand the meaning of most SC questions; For CR, when read each choice, I just read it but cannot link, in my mind, its relationship to the argument (In mock tests and practice, I was always able to do so)). I drank 1 Red Bull and about 35 gram of chocolate in each break (so 2 Red Bull in 70 gram of chocolate in total), so I don't think I did not get enough energy during the test. But after each real test, I felt nearly exhausted, while it was not the case after mock tests. I don't know it was because of energy depletion or because of psychology when seeing the low score.

Now, I still want to give the 4th try. I believe I could get better Verbal score than V31 because I retook GMAT Prep tests and knew that the V36 level is within my reach. But do you believe if I could do it better? And how could I make it, by improving the stamina (I already ate, exercised, and slept very well), or by learning any new strategy (probably try a new Prep course). Also, I have a good reason to take the 4th test: the Adcom from my most favorite school encouraged me to retake one more time to get 700+ so that I could get a fee waiver (175$), and he/she extends the R2 application deadline for my case by 15-20 days. I was inspired by this encouragement. But I also plan to apply some other schools in R2, so taking the 4th attempt now could negatively affect my apply plan.

Thank you so much and any advice is highly appreciated!