What materials are you using to study SC? Are you using our SC strategy guide? I'm guessing yes because you ask about doing the basics versus the later chapters in the book.
It is the case that you should not move on to the advanced chapters unless / until you feel that you have mastered the main material in the equivalent chapters. (eg, don't do the advanced parallelism material unless / until you feel very good about the main parallelism material.) If you are shooting for a 650, you do not need to know all of the advanced material. (If there are some areas in which you're good, then go ahead and try the advanced materials in those areas.)
There are multiple different possible problems that you could be having (and maybe you are having multiple problems). When you do the In-Action questions at the end of the chapter (the non-GMAT format sentences), do you struggle with those? Are you so-so? Do you mostly understand them? If you struggle or are so-so, then you are having trouble with the grammar itself. If you mostly understand them, then your trouble may be more about how the GMAT writes the sentences and how you have to think / work in order to be able to answer them.
If you are having trouble with the grammar itself, then you may want to take a class, work with a tutor, or spend a LOT of time on the forums, reading through instructor explanations and asking additional questions yourself.
It's also important to study the OG questions almost as though you're trying to learn how to write them yourself. How did they write that tricky answer B - what trap did they include and how did they get you to fall into it? How did they get you to cross out correct answer C, even though it was right? How did they disguise some particular grammar issue? Etc.
These articles can help you learn how to read and analyze SC questions:
How to Read:
http://www.beatthegmat.com/mba/2010/06/ ... on-problemHow to Analyze:
http://www.beatthegmat.com/mba/2010/03/ ... c-questionI think it would also be beneficial for you to do the analysis described in the following article on your most recent MGMAT practice test:
http://www.beatthegmat.com/a/2009/09/23 ... tice-testsThis can give us more precise data on your strengths and weaknesses (in everything, not just SC!) and that will help us to address your issues more specifically.
(Note: it is most beneficial to do this analysis on a test taken under official testing conditions, including essays, length of breaks, and so on. If your last test was not under official testing conditions but, say, the one before that was, then review the one that was under official testing conditions. If you have deviated a great deal from official testing conditions - say, skipping the essays, taking 20m breaks, and using the pause button - then I would recommend that you take a new test under strict official conditions. Then do the analysis on that test.)