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I'm looking for some advice on how to proceed...

Background: I graduated in 2010 from UG and have since done a one-year AmeriCorps term and am currently working full-time. My LSAT diagnostic score was a 158. I have been studying for the LSAT seriously (on average 20-25 hrs per week) since August, originally preparing for the October test. In preparation for the October test, I used the Powerscore LRB and LGB. About 3 weeks before the October test, I realized that I wasn't going to be ready, and decided to postpone until December. I continued to prep during the month of October using the MLSAT LR book, of which I have read the entirety. Mid-way through that book, I took a few PTs from earlier years (30s-40s) and saw my score jump into the upper 160s-lower 170s.

For the past 10 days, I have been taking PTs (all strictly timed) that have been more recently administered (55-62), but can't seem to get anything higher than 164. My major issue is timing, on RC I can never get to all 4 passages, which typically leaves between 6-9 questions not even looked at. On LR, I typically am only able to hit about 18-20 of the questions. On LGs, I have pretty much no issues at all with speed or accuracy. When I take LR/RC sections without timing, I get maybe 1-2 wrong at most.

My goal was to get into one of the T14, obviously more towards the top if possible. My GPA is only a 3.54 so I know that in order for that to be a solid possibility, I must increase my score to a 170+.

So, my questions are:
-- Should I withdraw from December or go ahead and take it to see how I do?
-- Do you think the fact that I haven't seen any major improvement basically beyond my diagnostic after using both Powerscore and MLSAT demonstrates that more than likely I am just not capable of getting into the 170+ bracket?

Thank you for any and all responses! Any suggestions on how you think I might be able to improve (like take a class, private tutor...anything) are greatly appreciated as well!

If you can score in the mid 160s, you can score in the 170s. I would argue for you to shut it down for a while and get ready for the June exam. Take a 3 or 4 month break, then hit it hard during spring. Looking at what you have written above, I think your supplements are just fine, but I would guess that you are relying on innate talent to score well, rather than truly learning the test. It takes a lot of directed and sustained effort to get into 170+ for most people. That means taking a lot of time to understand why you are missing questions. It means seeing a pattern in your missed questions and systematically working to eliminate weaknesses. Taking test after test and hoping to pick up some things will only get you so far. Now you have to be a machine and attack the test. You can do it, but it is a lot of work and requires a lot of reflection on your abilities.

Thanks for your advice! I think you're right. I have been trying to follow their methods and when I do questions slow, I feel like I'm able to apply the methods, but when I take the test itself, I just resort back to exactly what I used to do. I'm not against putting in the hard work to get my score where I want it to be and I'd rather take the time to really dedicate myself to getting and score, knowing that I gave it my all than settle on a score that is short of my goal. Even if I never reach my goal, at least if I go back and really get to know the test like you said, at least I know I am exhibiting my full potential when I get my score.