When you say you're not able to do 700-800 level problems consistently if you get a couple right - that's what happens to everyone! (Unless you're scoring 750+.) Theoretically, in order to get a 700, you don't have to get any 700+ questions right; you just have to get all of the questions up to and including 700 questions. (I say theoretically because the test never works totally "perfectly" - you'll get some higher level questions right but some lower level ones wrong.)
As Jonathan said, make sure to concentrate on the level you're at right now (plus a bit). Basically, these questions build. If you aren't consistent at, say, the 650 level, then you shouldn't be worrying about the 700 level, because you're not going to lift yourself there if you keep getting 650-levels wrong. Plus, if you're currently around 600 and work to master the 650 level, that will help lift you when you try the 700s. But if you skip directly to the 700s, you're going to find that a much harder transition.
Your most recent score was 590, so you should be mostly concentrating in the 600 to 650 level for now. You should also look at the data to see where you might be scoring even lower (that is, your weaknesses, the areas in which you're maybe only getting things right consistently at a 500 or 550 level). For those, you'll have to start a bit lower. Look for problems that you either (a) can do correctly but can't do in the given time limit or (b) can't do correctly but you feel like you're almost able to do them or you should be able to do them (an honest "should be able to," not a wishful thinking "should be able to").
I agree with Jonathan re: the 1000 series - I don't like those problems. Some of them are fine and are very GMAT-like, but there are also a lot of errors and bad problems. Make sure you time yourself when you do the problems. Your accuracy stats for OG mean nothing if you aren't holding yourself to the average timeframe.
I would also do some amount of balanced studying between your strengths and weaknesses - maybe 60% of time on your weaknesses and 40% of time on your strengths. You have more room to improve on your weaknesses, but it will be easier for you to improve on your strengths, so you want to do some of both.
And I also agree with Jonathan re: doing OG again. I wouldn't just do the ones I got wrong - I'd do them all. And I'd try to answer the questions I listed in this other post when I review the problems:
http://www.manhattangmat.com/forums/reg ... t5927.html