Hi mlehlali,
The 'swings' in your Quant and Verbal Scaled Scores are likely due to the number of 'lucky' and 'unlucky' guesses you take (especially in the Verbal section - when you might often be narrowing the answers down to 2 choices and in DS - in which not doing enough work will make you think that one of the Facts is sufficient/insufficient when it's actually not). Thankfully, the GMAT is a consistent, predictable Exam, so there are skills that you can develop that will minimize your need to 'guess' too often.
Before I can offer you the specific advice that you’re looking for, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How many hours do you typically study each week?
2) What study materials have you used so far besides the books that you mentioned?
3) On what dates did you take these 3 CATs? Have you taken any of the Official GMAC CATs?
Goals:
4) When are you planning to apply to Business School?
5) What Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich