That could be one factor, yes - the essay and IR sections use up a certain amount of brain energy, so that would impact you on the later sections of the test. If that's the case, it would mean that your prior scores weren't actually representative (because you weren't taking the full test - only quant and verbal).
Still, that's an awfully large drop, so I'm going to guess that there are also time management issues going on. The GMAT is a "where you end is what you get" test, so if you start making more mistakes towards the end of a section because you're tired and/or running out of time, then your score will go down. The scoring mechanism isn't an average of your performance, so it doesn't matter how high you were earlier in the section - where you end is what you get.
I would reschedule, yes - until you figure out what's going on, you don't know whether your real test score will be at the higher end or the lower end of that range. Better to figure that out first.
Assuming that you do have timing issues: it typically takes people about 4 to 6 weeks to address timing issues (possibly longer, depending on the severity). So give yourself enough time to get this worked out.
The next step is to analyze your last MGMAT CAT (unfortunately, GMATPrep doesn't give us data to analyze). Use that analysis to think about how things were or felt different during the GMATPrep test.
Have you already done your Post-Course Assessment? (This is a 30-minute phone call with an instructor that comes with the program - the instructor will review your tests and help you come up with a study plan.) If not, send an email to
gmat@manhattanprep.com to request the Post-Course Assessment (PCA).
You can also continue to get advice here, of course. First, read these two articles:
https://www.manhattangmat.com/blog/inde ... lly-tests/http://www.manhattangmat.com/blog/index ... -the-gmat/Think about how what you've been doing does and doesn't match up with that and how you may need to change your approach accordingly.
Then, use the below to analyze your most recent MGMAT CATs (this should take you a minimum of 1 hour):
http://tinyurl.com/analyzeyourcatsBased on all of that, figure out your strengths and weaknesses as well as any ideas you have for what you think you should do. Then come back here and tell us; we'll tell you whether we agree and advise you further. (Note: do share an analysis with us, not just the raw data. Part of getting better is developing your ability to analyze your results - figure out what they mean and what you think you should do about them!)
(But I definitely recommend the PCA first if you haven't already done that!)