Hi simplicius,
Study plans vary from person to person, so what worked for someone else might not necessarily work for you. Taking an "all Quant, then all Verbal" approach is probably NOT in your best interest though, since it would force you to remember all of that Quant for a full month before you really practice it again. You'll likely find it beneficial to do "some Quant and some Verbal" each week.
I suggest that you put together a mix of Quant and Verbal study for the next 1-2 weeks (don't try to hit every category though) and let's see how you progress.
One of the 'issues' with IR is that there wasn't a 'reference' point for that section when it first appeared (what did an IR score really 'mean?') - and there was no obvious score 'connection' to how a Test Taker performed in any of the other sections. There were, and still are, high scoring Test Takers who 'bomb' the IR section, so you have to ask what that might mean.... Does that mean those people are unworthy to attend a Top Business School? Or does it mean that the IR section is biased/unfair in some way? Those same issues/questions still exist today, so until someone comes up with a (relatively) foolproof way to evaluate IR as part of the larger application process, many Schools simply choose to ignore that score (and focus on the areas that they DO know how to evaluate - GMAT score, work experience, essays, interviews, etc.).
GMAT assassins aren't born, they're made,
Rich