50 points in a week = unlikely to happen - most people would need more time. So, yes, if I were you, I'd postpone.
Why did your math score drop so much and your verbal score go up so much on the middle test? That kind of fluctuation often indicates timing problems. If so, you've got to fix those, or you're at risk of a big score drop on test day (because, when you have timing problems, you can never know when both scores will fluctuate down
at the same time!).
Read these:http://www.manhattangmat.com/blog/index ... to-win-it/http://www.manhattangmat.com/blog/index ... anagement/
Also, you mentioned that your score has been stagnating for about a month. That's also a sign that you likely have timing problems. See below.
Next, you have taken 3 practice exams in 3 days. Stop. :) All you're doing is getting the same data and wasting practice tests. Use the below article to analyze your most recent MGMAT results:http://www.manhattangmat.com/blog/index ... ice-tests/
Then come back here and share that analysis with us. (Note: share your actual analysis, not just the raw data. We want to know what you think it means.)
You're right that the higher you go, the harder it gets to go even higher. Analyze your situation using the various articles above and then share your analysis with us; we'll help you figure out what to do!