I liked the Princeton Review Verbal Workout for sentence correction. I do not have the
MGMAT for Sentence Correction but I am sure that would help you as well. Use the
Error Log for Sentence corrections. Once you start using it you will begin to see a pattern like the 2-3 splits, incorrect idiom usages, misplaced modifiers, subject verb agreement etc. A few sentences may just sound correct to your ears and hence you could choose those.
Error log may take a long time to work on but nevertheless crucial to bring your scores up.
I got a 490 on my GMAT that I took last time and I scored a 35 and 21 on Q & V respectively. Sounds like we both are in the same boat. You are not alone
Good Luck