I edited my previous post because it wasn't clear the message I wanted to convey.
If you've finished OG's, Verbal Review, then do the 1000 sc,verbal tests (21-30),
MGMAT question bank. What I meant is that, in my point of view, it's better to practice official problems -- specially sentence correction problems. If you've exhausted official problems, understanding the concepts tested, then move to other resources such as 1000sc or verbal tests
Do GMAT prep problems few weeks before you GMAT. Those problems are more difficult than the ones in the OG's and are the closest to the ones you'll see on the official test.
Regarding the "some of the concepts" issue, for instance take problem 184 in the 1000 sc. That problem has been posted several times because people don't know, as I didn't before, that "if only because" is a correct construction. None official problem , as far as I know, has tested this construction and you also don't have an official explanation about the use of "on the bases of" and "on the grounds of". If you look for this problem in forums everybody has a different opinion about this question and most people select A or B as a the correct answer choice. Of course that is a good problem to learn a new concept, but might be a concept that will never be tested on the GMAT.
Good luck with your studying