Alternative approach to GMAT SC? MGMAT not helping much.

I've been pretty happy with my progress overall. My tests have gone:

1st MGMAT, 7/6/10: 630 [Q36/V40] (didn't have enough time to finish last 1/3 of quant)
2nd MGMAT, 8/17/10: 690 [Q48/V36] (had just finished all of the MGMAT math books)
3rd MGMAT, 9/8/10: 710 [Q48/V39] (finished MGMAT verbal books up to Advanced SC)

I wasn't too happy that my 2nd test's Verbal score went down, but I hadn't gone through any of the MGMAT Verbal books. The MGMAT books were very helpful in quant, so I thought I'd boost my score through some studying.

Well... it turns out my CR and RC scores are very good -- only missed 3 CR and 1 RC question on this last test.

On the other hand though, I totally bombed the SC. Only got 4/15 right... 2/5 correct on 600-700 difficulty, and 2/10 (!!) on 700-800 difficulty. This is in fact worse than my first test (8/15 right, 700-800 difficulty), and second test (1/1 right on 300-500 difficulty, 3/4 right on 600-700 difficulty, and 3/10 right on 700-800 difficulty).

I haven't covered the advanced SC section of MGMAT SC, but it's a little shocking to me that I'm doing so bad. I generally do okay on the MGMAT SC problem sets, and I've been doing the recommended OG Verbal SC questions (saving OG12 for the final push), in which I've generally scored 70% or better. But when it comes to the actual test, I'm forgetting all the little rules (which vs. that, like vs. as, pronouns, idioms, etc.). I think the MGMAT only trained me to spot the errors I'd been studying in the previous chapter.

It doesn't help that all this technical grammar jargon is still overwhelming me. I'd prefer honing down my "grammar ear" or "instinct" over pointing out specific past participles, relative pronouns, transitive verbs, etc. Even the answer explanations blow by me -- I have to pull out the MGMAT Grammar Glossary just to figure out what the answers are saying (I'd prefer a simple "you don't use this word after a description" over all the technical terms). I'm having no problems eliminating the 2-3 choices that "sound" bad -- incorrect verb tenses, run-on sentences, awkward wordings -- but I'm always left with the 2-3 choices that sound fine and boil down to correct idiomatic usage or those picky details I listed above. And apparently I'm almost always choosing the wrong one.

At this point my course of action is to finish up the MGMAT Advanced SC, re-read my Kaplan Premier and Kaplan 800, and trudge through the OG12 and whatever other question sets I can come across. But I think I need to turn to another book or another resource in the few weeks I have left before my test (late Sept/early Oct, haven't set a straight date yet).

Can anybody recommend some strategies and resources?

P.S. I'm a native speaker. I think what might be screwing me over is my job and background -- one of my primary job responsibilities is to write copies and ad material for my company. Everything I write needs to be concise and have flow. I'm also a big fan of William Zinsser's writing books, and these long-form, technical, detailed sentences are really throwing me off.

P.P.S. Oh, and if it helps, I'm consistently finishing the Verbal section with lots of time left on the clock. Like 10 minutes or more. Probably a holdover habit from rushing through the Quant section, where I'm always left rushing through the last 5-10 problems.