Hi, there. I'm happy to give my two cents.

First of all, yes, in the OG, in that bank of 170+ DS questions, the questions get harder as the section progresses.
I'm curious about the mistakes you are making --- you say, "all sorts of reasons ranging from silly to conceptual," but you also say that you are not having this problem on other sections, including the Problem Solving, which tests the same mathematical concepts.
How well do you understand the structure of the question itself? Is that the issue? Or do you find that, for a given mathematical concept, you can work with it in PS but not in DS? I'm trying to understand your difficulty with DS.
Here are a few free practice DS questions, all rather difficult.
https://gmat.magoosh.com/questions/951https://gmat.magoosh.com/questions/1026https://gmat.magoosh.com/questions/999https://gmat.magoosh.com/questions/1037Each one should be followed by a video explanation of the question. In the paid section of the
Magoosh site, we have a series of 16 videos detailing strategy on the DS section. You can find out more following the links in the signature.
Mike