High variation in GMAT official preptest scores

Had you encountered the GMATPrep 1&2 problems prior to attempting the test? Since 1&2 are free tests, the problems are more easily available and tend to pop up often - having solved the problems before taking the prep test could have inflated your score.

Test 3 pool of questions are very representative of those on the official GMAT and Test 4, I would argue, is slightly easier. [I got 770 in Test 4 and breezed through the quant when I had had encountered at least a few tricky problems on other Tests.]

Another contributing factor could be your mental state. It's easy to underestimate it's importance, but being in the right state of mind is absolutely critical to scoring well on the GMAT; even the Prep tests. I took my first official GMAT under extremely stressful conditions and scored a 640. [My 3 most recent Prep Test score prior to the attempt were 730, 760 and 740]

Bottom line is that GMAT Prep tests don't have as large a variance as you're suggesting - there certainly must have been other factors that you can diagnose and fix.

Side note: You've got a great Quant score, but if Verbal is your problem then you must must MUST reinforce your basics. It may seem a drag since you're only two weeks away from the test, but the ONLY way you improve in SC and CR is by reinforcing basics. You cannot improve your verbal score by solving more problems. My Verbal score went from V42 to V46 in 2 weeks of study by just filling gaps in my fundamentals [essentially by watching Ron Purewal videos - you'll find links to most of the videos on this forum somewhere.] It should help you get your score to V37 or more. Feel free to PM if you have questions regarding Verbal Strategy.