Tim,
You can't trust your ear while giving GMAT. The logic used in GMAT CR is bit interesting, so you need to give some time on that.
Try these books:
1/ Power Score CR bible. Don't read it all. Let me know if you want me to tell you the page numbers you should focus on. GMAT authors have very typical way of thinking in their GMAT centric way. If they say something is hapening due to something, they mean that it can't happen due to any other thing. Such sort of very important info you can find in this book.
2/ Try Kaplan Verbal Supplement for RC: It's very crisp and it tells you to focus on the gist of the passage and to watch for changes in the tone. Passages are very easy to understand if you see what they really are. Authors have a very set patters in their passages.
General pattern is something like:
1/ They introduce a theory in first paragraph, and then they tell that there are alternate theories. They explain these theories.
2/ Watch out for conjunctive adverbs: furthermore, however, etc.
3/ Watch out for things such as examples. Examples elaborate the idea that was presented before. So if you don't understand what was in the previous sentence, you can see the example and try to deduce it.
You can drastically improve your quant score. Go through the maths books by Manhattan. Though they may take time as they are bit lengthy.