I was having the same problem till a couple of weeks back, but off I am touching the 700 mark on GMAT prep. Never could do better than 680 on
MGMAT.
What I could decipher from GMAT prep is that,in V, we need to get less than 10 Q's wrong to score anything around high 30's. Like 39 or 38. Anything above that, GMAT prep penalizes very badly, with the verbal dropping down to less than 35.
In Q, I found out that the more hard questions we get right, though we get some easy ones wrong, we get a good score of 48-49. Once I got only 9 questions wrong, in two tests, in Q and got 49 on GMAT prep and the other time I got some 13 Q's wrong, but still ended up getting 49 in Q.
What I analyzed was that, in the third case, though I got some easy ones wrong,silly mistakes; I got some toughies right, some combination's and probability and a couple of co-ordinate q's right.
So my personal suggestion, high accuracy in Verbal and getting hard q's right in Q will take you past 700 on GMAT prep.
I am still to give my GMAT, so I am not yet THE guy to give suggestions, but still, my personal experience.