from 750 gmatprep to 640 on actual gmat

Sorry to hear that--it happens. Scores fluctuate, and nerves / health / luck of the draw (25% pretest questions on the real thing, no pretest questions on GMATPrep) are big factors. As a GMAT tutor, I've taken the GMAT 4 times, and my scores have ranged from 700 to 770. In general, I would say that scores tend to fluctuate about 80-100 points based on random factors, so going from 750 to 640 is certainly not unheard of.

Taking the GMAT is essential for those of us who want to continue to challenge themselves from a student's perspective, and for staying up-to-date with changes to the test, which continues to become more difficult every year. What I have found from taking it frequently (4 times and counting!) is that there is undoubtedly an element of chance involved in getting a 700+ score. That's why the GMAC had no choice to institute an 8-test limit...students were taking it again and again, in an effort to maximize their scores. By catching some lucky breaks on those 50/50 guesses on Verbal, and "punting" on Quant questions that turn out to be experimental/pretest questions, you can raise your score by up to 100 points. That's how I got my 770--I just happened to have a good day in both sections, and yes, I got a little lucky.

What did you get on IR? Maybe you burned yourself out at the beginning?

My test-prep motto: "Perfection is attainable, but not maintainable."

Keep trying,
Brian

p.s. Don't forget to cancel your score, if you haven't done so already! You can always reinstate a cancelled score for a fee, but you can't cancel it if you miss the 72-hour window.