I think you are doing too many problems! Try this:
1) Pick 5 SC problems and give yourself 6-7 minutes to complete the set. Make sure you have recorded an answer for each problem ,even if it is a guess. Your goal at this point is to maximize the total # correct, not to master any one problem.
2) DO NOT check the answers yet. Go through each problem untimed, and attempt to get as much as possible out of the problem. At the end of the process, adjust your answers as needed. Here are some questions to ask:
*What topics are tested in this problem, and how can I tell?
*What problems does the correct answer fix?
*What is wrong with the 4 wrong answer choices? (There may be several problems with one choice.)
*Were there any traps? (For instance, a split that doesn’t really matter, or a correct choice that is written in awkward or unfamiliar language?)
*Did I eliminate any answers without a clear or sensible reason?
*Are there any important elements of this problem that I missed at first? Why did I miss them, and how might I catch them next time?
*What was the most tempting wrong answer, and why? What is the difference between that answer and the correct one?
3) Check your answers. Only count the problem as correct if you got it right during Step 1. If you missed the problem even after extensive review, see if you can catch your mistake. Was there a split you were unsure about, or does this missed problem come as a complete surprise?
4) Check the explanations and re-review as needed. You may need to ask some of the above questions again.
The main point here is . . . don’t just settle for confirming that your answer is right. Try to determine the reason for *every* single difference between the right answer and the wrong choices. You may need to compile a list of questions and use the forums for support. If you are doing this kind of thorough review, you shouldn’t have time to do 70 questions in one day. You could easily spend an hour or more on 5, and it’s worth your time to do so. Sure, every SC problem has something different to it, but what’s much more important is to familiarize yourself with the way the GMAT plays with language, and to develop a reliable approach for recognizing important issues and eliminating quickly and accurately.
I’m assuming here that you have already done some sort of prep, and have knowledge of many of the grammar rules tested on the GMAT. Naturally, if that’s not the case, you would benefit from covering that sort of material, using our SC book or any other source of GMAT-specific grammar instruction that you have available.