First, don't panic. That's the last thing that you want to do as the test date approaches.
As you saw repeats on the GMATPrep, your score is inflated. Seeing that 700 score, while tempting, doesn't actually represent your true performance on that day, due to the old questions. The last two
MGMAT tests (what are your other scores, exactly, by the way?) show a good performance, so let's use that as your benchmark: 650. A yet unexplained drop of 50 points, instead of 100 points, doesn't seem so bad.
As you hinted in your post, Kaplan had a reputation of administering very difficult CAT's. Nowadays, however, the company altered its exam format in order for the tests to more accurately reflect the actual GMAT. But perhaps there's still some room for improvement in that area.
How many CAT's have you done? The two GMATPrep tests (along with repeats), the six
MGMAT tests, and some Kaplan tests sounds like too much. You might be burning out on them or practiced with too many in a short time period. If so, then it's probably time to take a break from the CAT practice. And as this is the sole "bad" performance, you shouldn't worry about it that much. Your other scores are better and perhaps more in line with your true capabilities.
So, the main culprits appear to be the inflated score, which may have spoiled you to some degree, and possible burnout associated with the tests. This lead to an over-reaction and some level of panic. Again, the best remedy is to take some time off from the tests and relax.