Unfortunately, there are no great answers to your conundrum.
1. The test fluctuates in terms of where it allocates its difficulty.
Some tests have a legendarily hard Games section. Some have a super hard RC section. Etc.
So we hope/expect that our overall score doesn't fluctuate too much, but we find it normal for there to be some section-by-section fluctuation.
If we're faced with a harder than average RC section, then we should be doing better than average somewhere else in the test. In case you notice yourself feeling like a bad RC section has "ruined this test" WHILE you're still taking the test, you should remind yourself of this mantra. When one section goes poorly, it's an opportunity for the others to go better than usual.
2. Modern RC feels harder (to me, at least)Standardized tests get harder over time. They have to, or else the population of test takers, who are getting better at the test compared to their peers from years prior, would start scoring too high.
In other words, since we all get better at LSAT by doing old LSATs, we profit from the wisdom of past tests, which means that we're better at LSAT than were the people who took those LSATs. If LSAC gave us a new test that was comparable to an old one, the smarter population would score too high. So they have to keep making the test slightly harder over time in order to keep the same distribution of scores happening.
RC in the PT30s is more loveable because there's a clearer correspondence between a correct answer and the supporting lines from the passage. Also, the passages tend to have more of a narrative arc, so it's a little easier to find the big ideas.
Modern RC can have tougher vocab, but it also has more descriptive passages that lack clear thesis statements or lack an obvious opponent against whom we can measure the author's purpose / position.
Mostly, though, I find that there's a certain looseness in language in correct answers that is now tolerated that used to be out of bounds. I find myself a few times per section thinking, "I don't have ADEQUATE support for this answer, but I have MORE support for this answer than for any other. I guess I'll pick it?"
Even if I end up being right about that answer, it doesn't change the realtime feel of my losing confidence and time as I agonize over why I can't find any answer I feel great about.
So mostly you just have to accept that what you're scoring on RC in the 70's / 80's is a much more reliable indication of how you'll do on an upcoming test than what you're scoring on RC in the 30s.
Where you're at in the 50s, there are still some legendarily hard passages (British common law jurisprudence .... riddled basis of attraction .. re-seeding the topsoil with native herbs / microorganisms ... etc.)
Good luck (to all of us).