First of all I would like to say that 650 is a decent score. so don't beat yourself up for that. Depending on the schools you are targeting it may be considered a competitive score.
Clearly you need some work to do on verbal. From what you described I can conclude that the methods you used to prepare for verbal are not working. so you need to reinvint the way you are preparing for verbal.
I will try to address the apparent discrepancy in your GMAT Prep score and the actual GMAT score. On your GMAT Prep tests did you skip the two essays? Because part of the reason why you scored low in verbal on the actual GMAT may be that you are exhausted towards the end of the exam. Lets not kid ourselves but GMAT is a pretty long test. It certainly happened to me the first time I took it. So if you have been skipping the essays in the prep tests then stop doing that. Next time you take a prep test make sure that you write the essays and then see how you perform the verabl section. If you scored poorly that is an indication that your losing your energy or concentration towards the end of the exam.
If you think that didn't play any role in your not so stellar verbal performance then you need to stop doing what you have been doing. Clearly it didn't work. you seem pretty confident that you did well on CR & RC. So that leaves us with Sentence Correction. take 10 questions from the OR at random and answer them. categorize the questions you got wrong and read those categories' chapters from the MGMAT SC
book. Another exercise to test your understanding of the SC topics is to read news article and try to identify the GMAT SC topics. For example, everyday during your lunch time print two or three news articles from good news websites such as NYT (notwithstanding its left leaning
), Reuters, The Telegraph (UK) & The Gaurdian (UK). In each article underline the parts of the sentences which have parallelism, comparisons, subject-verb agreement and tenses and other GMAT SC topics. Identify each one and cross check whether they conform to the rules described in MGMAT SC
book. By doing this exercise you will train your brain to read the way GMAT wants you to read. You will be constantly reviewing those topics and your fundamentals get stronger.
Hope this helps. if you have questions or more information to share let me know.