GMAT Verbal Section | Verbal score decreasing with studying

I'm in a puzzling and frustrating situation. I took my diagnostic test in GMATPrep about a month ago and got a 760 (Q48 V48) which was pleasantly surprising. I finished verbal with something like 15 minutes to spare and only missed two questions. RC and CR come pretty easy to me, so SC was my weakness. This was all just based on my inner ear (I'm a native speaker) and what little formal grammar I could remember from my freshman writing class.

Since then I've been studying w/ Magoosh and the Manahattan
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