I'll start by saying that nobody needs a 750 and I honestly believe it's a waste of time to go for that kind of score. (Unless you want to teach for us. In which case you need a 760. :)
If you turn in the exact same application with a 710 vs. a 750, that's not going to change your chances of getting in. Either score will make the school think "All right, you can handle the work here!" and then they'll make the decision based on other aspects of your application.
Okay, now that I've gotten that out of the way... :)
Your verbal score right now is in the mid to high 20s, so we're going to have to do this in increments - I can't give you a prescription today that will take you all the way from here to a 700+.
You had a 29 on test 2 and a 25 on test 3, so I'd like to know what the differences were that led to that drop in score. You mention timing. Did you time yourself during the GMATPrep test? (If not, for future, make sure you set yourself up to be able to track your per-question timing whenever you take GMATPrep.)
Still, you already know you have timing issues, so you can start there.
Timing is really a consequence of mindset: if you are hanging on to certain problems for too long, then your problem is that you're still in the school "I have to get everything right" mindset. You'll need to break that pattern. Read this:
http://tinyurl.com/executivereasoningIn fact, read it once a day until you've really internalized the message that that is how the GMAT works.
Next, start with section 4 of the time management article. Build your 1-minute time sense. Then use that time sense to help guide you as to when you should keep going and when you should guess and move on.
For SC: If you don't know exactly what you're doing by the 1-min mark, guess and move on.
For CR: If you don't know what question sub-type you have and/or you don't fully understand the argument by 1 min, guess and move on.
For RC questions:
Main Idea: treat this like SC
Specific: treat this like CR. By 1 minute, you should: know what Q sub-type was given; have found the relevant text in the passage; have re-read / understood that text. If not, guess and move on.
I'm going to guess that at least part of your score drop from test 2 to test 3 was due to hanging on too long on some problems - so if you fix that, your score will go back up just because you'll allow yourself to address the problems that you actually do know how to address.
Next, we need to dig into the data to figure out exactly what the issues are that are causing you trouble on verbal. You aren't going to get to a 700+ by working only on one of the three question types. Use the below to analyze your most recent MGMAT CATs (this should take you a minimum of 1 hour):
http://tinyurl.com/analyzeyourcatsFigure out your strengths and weaknesses as well as any ideas you have for what you think you should do. Then come back here and tell us; we'll tell you whether we agree and advise you further. (Note: do share an analysis with us, not just the raw data. Part of getting better is developing your ability to analyze your results - figure out what they mean and what you think you should do about them!)
For RC in particular, think about WHERE you are having the most trouble. On the first read-through of the passage? Are you getting the main idea but getting lost in the details? Are you getting so lost in the details that you're not even getting the main idea? Or is the read-through fine but you're getting tangled up in the answer choices? All of the above? For certain question types? Etc.
Also, read the below and make sure that this is actually how you are studying - including RC! You get better by analyzing the types of questions the test gives you:
http://tinyurl.com/2ndlevelofgmat