What to do? Plummeting scores in 3 attempts

My first attempt at the GMAT left me with a 670 (Q44,V38). Nine months, a Princeton Review book and the Kaplan GMAT CD later, I scored 660 (Q36,V45). This was a bit unexpected, as I spent the greatest amount of time working on my quant score. Immediately after the 660, I purchased a Kaplan Math Workbook. Over the last 3 weeks, I completed every problem in the book. My math comprehension has grown tremendessly. On the way home from work, my wife would read problems to me (many in the advanced sections), and I would work them out in my head and explain them to her. Yesterday, I took the GMAT for what I hoped would be the last time. With my new math knowledge, and hope for a repeat of the V45, I expected a score in the 710-730 range. Instead, I was blindsided by a 600 (Q37,V35)! How can this happen? Unlike my other attempts at the GMAT, I actually felt like I knew the answers to the math. I guessed on the last 3 or 4 questions because I didn't manage my time well (I checked my work on the first 25 problems and noted on my scrap paper whether I "knew" I had the answer right or if I was just guessing). How can my score drop so much after I put so much effort into preparing for the exam? What should I do now? Will taking the GMAT a 4th time look bad on applications? I'm really not satisfied with a 670, but I'm not sure that I want to risk scoring below 670 again. My understanding is that schools only have access to the 3 most recent attempts.

Please help!

Thanks!
Christopher