The fact that you've gotten around 350 on three different administrations clearly indicates that you are lacking either on conceptual clarity or application skills or time management skills or the exam temperamemnt or most likely on all the four afore-mentioned areas. I think at this point, your performance has a lot less to do with Kaplan but with your ability to perform on this exam . Since you've consistently been getting bad scores, what you need to do is -unlearn whatever you've learn till now because if you don't do that, your past learning may become a stumbling block if you try to implement new approaches of going about this exam. Secondly , focus on conceptual clarity first. How good were your concepts ? For example, could you spot the errors in the underline portion of Sentence correction questions without reading the options? Did you have strategies about how to tackle the various types of Critical Reasoning questions ? Did you practise enough of RCs and could you find any pattern about the errors you made.

Did you take any GMATPREP tests before the actual GMAT and if yes, how much did you get on that ?

As I am preparing for my third appearance, I can totally relate to your experience.

You haven't provided your split or preparation methods, so I am just giving what I didn't do in my previous attempts and trying to to do now.

1. There's a huge difference in going through some book/course/test without actually following what you read.
What I mean is, you read Manhattan SC guide 10 times and you know in and out. However, do you apply them during your test?
That was a problem with me. I know the rules(at least some them), when it comes to practice or GMAT, in the time crunch I will not even bother to identify what is the error in the original sentence. Will glance through and pick what I think is right. This goes for RC and CR too (cos I am struggling in Verbal).

So try to follow and apply the points you have read about in practice test. Do not worry about timing first. You can pick-up timing once you know you are doing right.

2. As many people/experts would say, all you have to do in practice is review. Do not see what the actual answer is, but see what the technique is. For example, in a CR stem you should know, the technique which you should follow. If the stem says a new method would be the solution to a profit/loss problem, the next thing you should think of is, whether the cost of the new method is included in the analysis. That is the (new cost + existing cost) < the selling price to make sure you get the profit.

3. Key is quality not quantity. Even if you spend 20 hours a day and didn't follow the approach, no point in quitting your job and preparing. On the same note, if you spend 2 hours a day for 2 months with quality review, you should be able to crack your score.

Finally, yes, it's time consuming and difficult process but that shouldn't throw you off! that's what head hunters are looking for.
Easier said than done! lol

Good luck to you.