Depthbuffer or similar bug?

Hi, I’ve got a very simple patch with a spread of textured quads. They take different forms, in a tube, in a (gaussian) ‘cloud’, anyways, that’s all fine, but i’ve got a weird problem I thought i’d fixed but it’s come back again.

At some angles, and with some arrangements of the quads, the black normally transparent background of the texture (coming from a text(texture.source)) becomes visible and obscures the quads behind it.
When this problem first appeared (it only happens with a small number of quads) I changed the depthbuffer of the renderer to D16 and it seemed to fix the problem.
Now the problem has come back even though the renderer is still set to D16.
Any ideas?

thanks.

try the node AlphaTest (EX9.RenderState), it helps in most cases. but not all. also read this article: transparency