On Win 7. No definitely no zombie process. Restarted. Cubase works like a boss, it's just Sibelius. And it's only when I use my manual soundsets, because the Sibelius Sounds are fine.
My channels are distributed sequentially, with no gaps in between. I suppose I could try to build it up again one by one, however I even ported my playback setup back to Sibelius 6 and the problem persists. Unless there is some kind of problem with this playback config being corrupted.