I think the reason why is one of, or a combination of, three reasons: 1) endurance, 2) timing/feeling rushed, and 3) question difficulty (harder logic games and passages tend to be one of the last two in each section). I'd recommend checking your timing for the earlier questions after 7sage's practice tests to see if it's an issue of timing. You can also look at the question/passage/game difficulty for the ones you're missing and see if it's difficulty.

If you think it's partially endurance, right when you finish a section, immediately grab another from one of the earliest tests and just do 5 more problems at random. It's like with running - you train at distances longer than you "race" for.

Hopefully this helps. I've had a number of concussions, so fatigue is a major issue for me, and it's helped me.

I also agree with the translation drill recommendation above! Read the stimulus, cover it up or close your eyes, and translate it back into your own language. Identify the conclusion, and the controversy or the loophole, etc

There are a million ways to test this kind of concept. Not sure why they choose topics like this...

@ a few things - Flex is much more variable so those jumps don't shock me. That being said, it probably is a sign you just aren't 100% comfortable with all the traps or problem types the test may throw at you yet. If you have a few days until you take it, I'd write down your main lessons and main mistakes your making, and then otherwise, just rest. It'll give your brain a chance to line out some of what you're doing right/wrong each test, and then give it another chance to process it. Best of luck.