Hi achbaa,
Many GMATers deal with 'fatigue' issues during their studies, so you're not alone. Test Day is a rather specific 'event' though, so the way that you build up your stamina over time is to properly mimic that event every time that you take a practice CAT. In simple terms, that means that you have to take each CAT in a realistic fashion (take the FULL CAT - with the Essay and IR sections, take it away from your home, in one sitting, at the same time of day as when you'll take the Official GMAT, etc.).
The 'swings' in your Quant and Verbal Scaled Scores are still likely due to little mistakes (and how frequently you make them or avoid them). "Review" is an exceptionally important part of the GMAT training process; your ability to define WHY you're getting questions wrong is essential to defining the areas that you need to work on (and the specific things that you need to 'fix'). As such, I'd like to know a bit more about these 2 CAT performances. While a full Mistake Tracker would provide a lot more information, there are some basic questions that you should be able to answer (and the more EXACT you can be with your answers, the better):
After reviewing each section of this recent CAT, how many questions did you get wrong....
1) Because of a silly/little mistake?
2) Because there was some math/verbal that you just could not remember how to do?
3) Because the question was too hard?
4) Because you were low on time and had to guess?
5) How many Verbal questions did you 'narrow down to 2 choices' but still get wrong?
GMAT assassins aren't born, they're made,
Rich