The overall scores you report are all within one standard deviation of each other - in other words, it isn't the case that your score dropped a lot on your second GMATPrep test. There are any number of things that can cause your score to fluctuate by 30 to 50 points even though your basic ability hasn't really changed. Some of those things are within your control (e.g., being well-rested) and some of those things aren't (e.g., randomly getting a few more questions in an area of weakness for you).
What about your subscores - what were your quant and verbal scores on the GMATPrep tests? Were they consistent with your MGMAT subscores, or is there some variability there?
Think about your practice test conditions. Was there any reason why you might have been a bit more mentally fatigued when you took the 680 test? Had you taken another practice test within 2-3 days before that test? Did you take it later or earlier in the day than you normally did? Etc.
In terms of assessing your performance in order to solidify your performance from here, use these articles:
http://www.beatthegmat.com/a/2009/09/23/evaluating-your-practice-testshttp://www.beatthegmat.com/a/2009/10/09/how-to-analyze-a-practice-problemhttp://www.beatthegmat.com/mba/2010/04/07/how-to-learn-from-your-errorsAnalyze every problem from the test (not just the ones you got wrong) using the 2nd article above. Use the information in the third article while reviewing problems you got wrong or problems on which you got lucky (you got it right but you didn't really know what you were doing 100%).
Also, just want to mention: the way you get better is to do the analysis described above. You don't get much better simply by taking a ton of practice tests. Just don't make the mistake that so many people have made in the past - don't think that taking a bunch of practice tests is what will help. At this point, with about 14 days to go, I would take a MAX of 2 more practice tests, and the last one would be taken 5-7 days before the real test.