Look at the data again. How's the timing? Are you having to speed up at later in the section because you're running lower on time than you should be?
If not, good - but, if so, then that's likely the main cause of your drop in performance at the end of the section. If that's the case, you need to know, because how you deal with a timing problem is very different from how you deal with a burnout problem.
Let me know if you realize that it's really more of a timing problem.
Then, take a look at the problems and ask yourself: why, specifically, did I get this wrong? (There may be multiple reasons.) Sometimes, it's just a hard problem and it wouldn't have mattered where in the section you saw it. Sometimes, you realize that you did know what to do, but you made a careless mistake. Read the below article to learn how to minimize your careless mistakes:
http://www.beatthegmat.com/mba/2010/04/ ... our-errorsAsk yourself what you can do in future to minimize the chances of making similar careless mistakes, especially at the end of the section. Do you need to be more systematic about transferring info from screen to page? Do you need to double-check that you're solving for the right thing? Do you need to make sure you're doing ALL calculations on paper, nothing in your head? Etc.
Now, you may be right that you are suffering from a stamina issue. Are you eating and drinking on the break between the essays and the quant? Are you standing up, stretching, walking around a bit? Are you actually taking the full break? (8 min, but spend the first and last minute walking around to simulate entering and leaving the testing room - so the break is effectively about 6m.)
Are you doing the essays? If not, you MUST do them from now on. If you're already suffering from a stamina issue, it will only be worse on the real test if you're not used to spending an hour writing essays first.
When you sit down to study, first make a 2-hour gameplan (everything you're going to do in that two hours). Take one 10-minute break in the middle. Otherwise, work hard the entire time - pretend it's the test. Do not eat or drink. Don't stand up. Don't check your email. Turn off your phone. Etc.
Periodically, set up random mini-sets of 20-30 questions for yourself, pulled from your OG book, or do GMAT Focus (
www.gmatfocus.com). Write two essays, then do the problems.
And if you are just feeling burned-out in general, then yes, take a day off!