First of all, it's hard to make solid conclusions from only a single exam. Even though you are dealing with a fair number of problems, there are a lot of variables that will impact your performance during the course of the test.
Here are some things you might consider:
-Timing (most likely): Are you spending too much time on the early part of the test and realizing about half way through that you are behind in the clock? Use timing charts to keep track of your progress throughout the exam. Also, you should be practicing
OG problems in a "timed set" format to get used to GMAT test timing.
-Mental (very possible): The GMAT can be a mental game. If you think you are starting to miss problems it can impact your mindset as you work through the rest of the test. You can't go back to any prior questions, so you need to do your best to get "amnesia" about the prior problems and focus completely at the current problem on the screen.
-Large Content Gaps (not very likely): In rare cases you can get stuck in content areas that are holes for you and you keep seeing problems that attack your weaknesses. You need to be a generalist to succeed at the GMAT - specialists in a few areas will be exposed.
-Coincidence (possible, but not likely): Sometimes you just a series of questions that stump you.
-Technical Error (not likely): I haven't seen this issue come up, but it is theoretically possible.
Do any of these seem to match your situation? I would recommend taking another test (GMATPrep,
MGMAT, or some other test of choice) to see if the pattern repeats itself.
KW