This is fairly normal, and it can be a serious problem on test day if your home practice is not realistic. For instance, if you practice without IR & essay, that extra hour can leave you truly exhausted by the time the verbal section comes along.
A few tips:
1) Make sure you are using your 8 min. breaks well. Get up and move around, use the bathroom, and have a snack/drink with some sugar in it. You can experiment with different snacks that are portable and easy for the test center, but that give you the necessary boost.
2) If you feel your energy or attention flagging at any point during the test, take a micro-break. Close your eyes for a moment while you take one or two relaxing breaths. Then look at the test again with fresh eyes. "Okay, what is my task?"
3) Make sure you are working for efficiency. Sometimes even when we are managing to time ourselves well, we work in an exhausting and inefficient manner. You can get more efficient by improving your ability at the underlying skills of the test (algebra, grammar, etc.), but it can also help to look for more efficient ways of thinking about different question types. For instance, on a Sentence Correction q, are you first eliminating the answers that have clear grammatical errors before trying to analyze more subtle issues of meaning and structure? In RC, are you avoiding some of the finer details of the passage, figuring you can go back and read them if a question requires you to? In CR, are you stopping to analyze the argument and predict an answer before moving on to the answer choices? Get in the habit of thinking about efficiency in your review of every practice problem.
_________________