Maybe a bit of a weird request, but I've had no success either looking on my own or asking anywhere so far, and I'm going a little nuts.

A few weeks ago I attended the Ali Project concert at Anime Boston, and there was a song I didn't know that I wanted to find. Based on the rest of their setlist, it was almost certainly from an anime that is fifteen years old or less, but I've gone through every anime OP/ED of theirs that I can find and still haven't come across it.

It was very much in the up-tempo wall-of-sound mode of their other anime themes (as opposed to the slower orchestral stuff they have on their albums and sometimes as insert songs and stuff), but there were bits in it where it abruptly slowed down and the music was more subdued, and the singer was sort of singing one syllable at a time all spaced out, if that makes sense? That's the only identifying feature that I can remember, but I doubt it can apply to many Ali Project songs considering that I have yet to come across any that do that.