Your RC fluctuations are normal, especially between older and newer tests. It is a common question in this forum. The more recent tests, I think, have somewhat trickier questions in that they are focused on the skills used in logical reasoning (you will see more questions asking you to strengthen/weaken, identify the assumption etc.). Your number of misses should even themselves out as you get exposed to more material but there still might be the occasional fluctuation. What is really important is to review your mistakes and see if there are question types or passage types where you get tripped up more often than not, and then to focus on those.
This will be my third time preparing for the test (I've already sat for it once and postponed the June administration) and I am focusing a lot on RC. I would recommend the method/philosophy in Manhattan's RC guide. However, I think that each person needs to experiment and come up with a method that works for them. What I found most helpful is to read the passage for structure, focusing on the role each element of the passage plays in the author's overall main point. I also underline important words and notate in the margins the various opinions and any definitions in the passage. I circle so-called key words that tell me where the passage is going like "but", "however", "some" etc., and box any names or dates so I can find them easily when the questions refer to them.
I hope this helps if only to give you some reassurance that RC fluctuations are normal. Unfortunately, it is the hardest section to improve but certainly not impossible so keep at it!