A 75 to 80 percent success rate in SC is pretty solid. But, if possible, it might help to read Manhattan one last time, or at least the sections in which you have the most trouble. I wouldn't read the CR again, though. Perhaps you just need more practice. GMATClub forums have hundreds of CR and SC questions along with full explanations. Try to answer ten per day, and fully explain your answer and why the other choices are wrong. This will reinforce the material, because you are practicing with more questions
and fully explainingthem. Even if you're basically copying a previous post, put it in your own words. This helps.
For RC, the best thing to do is practice. I'm a big fan of LSAT RC questions because the passages are very long and complex. The questions can also be more difficult. Once you're comfortable with LSAT RC, then GMAT RC shouldn't be a problem.
Also, be sure to keep an
Error Log and record exactly what type of CR, SC, or RC question you get wrong. If one question takes more than 2 minutes, then answer it anyway, but mark it as incorrect and redo it later. Use the
GMAT Timer for this. Whenever you are ready, time yourself to determine your base. After that, try to reduce the question time by a few seconds for each verbal practice session.
And whenever you have time, read some
GMAT Fiction. It helps, but in a very subtle way.
Here is some info about the
GMAT Timer:
gmat-timer-56014.htmltime-yourself-with-a-new-gmat-club-timer-v-96251.html