The mystery of good working music

I used to be able to work or read with just about any type of music playing in the background, but during the last year or so that ability have been fading rapidly. I'm now getting more easily distracted by a wider range of tunes - especially the kind where the vocals matter. Instead of focusing on the task at hand or the line of text I might be reading I get lost in the music. The task of listening to the lyrics seems to be getting discontent with the status of "non-essential" and keeps upping itself to "important". Fairly annoying.

So I've been resorting to a lot of instrumental music to get some work done. The 1:42 minute Braveheart theme has consistently proven effective and has found its way into the repeat-of-the-day section a lot, but it does get somewhat boring after a while. That has led me to experiment with different kind vocal-based music to see if I could find something that wouldn't consume 80% of my attention, and my level of succes seems almost random.

I've been listening to "Let Me Blow Ya Mind" by Eve (featuring Gwen) all day along with no problems whatsoever even though it has Eve rapping continuesly through the song. But almost any Everything But The Girl tracks throws me totally off even though they contain a lot less complicated lyrics. I'm puzzled...