Scored a 510 on Gmat in-spite of higher 600 scores on p.test

[ajurules]

I am a big fan of MGMAT, and have used MGMAT materials extensively in my prep.
I prepared for about 2.5 months , and took about 8 practise tests, in which one includes GmatPrep test. Below are my scores,

MGMAT #1 - Q45,V30 ( 620 )
MGMAT #2 - Q44,V30 ( 600 )
MGMAT #3 - Q41,V32 ( 600 )

-- At this point I understood that I had some issues with pacing, rectified that and took the next set of tests.

MGMAT #4 - Q47,V36 ( 670 )
MGMAT #5 - Q43, V38 ( 670 )
MGMAT #6 - Q40, V44 ( 700 )

I am a non-native english speaker, so I did give a lot of importance to verbal in the last few tests or so, which explains why my scores in quants went significantly down.
After this, I took a GmatPrep test, scores below,
Gmat prep #1 - Q46, V35 ( 660 )

I took all these tests with AWA, and with just one break for about 2 mins or so after Quants. In the actual Gmat, I took full 8 min breaks after AWA and Quants.

Keeping the accepted standard deviation in mind, which I believe is about 20 points, I had an expectation of 680 or so, and so I scheduled my appointment and took the test yesterday ( 17/08 ), and I got this score,
Actual Gmat - Q42, V20 ( 510 )

Disastrous I would say, but I can't think of what went wrong. As for quants I didn't make any guesses on PS, although I could have made some mistakes on DS, can't say for sure. Funny, as it is, I thought I was doing quite Okay in Verbal. I was quite disappointed with the score, and slept over it for most part of yesterday - but I have since got out of it and started thinking logically on 'what next'.
Would you have any suggestions? I am not sure now if GmatPrep is an apt indicator.

[StaceyKoprince]

Your quant score was roughly in line with most of your practice scores. Your verbal score really wasn't. From your writing, I can tell that you have a strong command of the language, despite being a non-native speaker, so I don't think that aspect of things accounted for the significant verbal drop.

Do the analysis described in this article, then come back here and share:

http://www.beatthegmat.com/a/2009/10/26 ... went-wrong

We'll use this to figure out what went wrong; if we can do that, then we can also figure out what to do to fix things.

FYI for future reference: the standard deviation on the official test is about 30 points. The SD on MGMAT tests is about 50 points. This is a +/- figure.

Your last test was the GMATPrep test on which you scored a 660, so your score range within one standard deviation was 630 to 690. SD means that you have a 2/3 chance of scoring within this range and a 1/3 chance of scoring outside of this range.