Just a thought, and maybe completely irrelevant... 

Why is the "Light" showing green in the D3D render settings?
Doesn't this mean there is AO somewhere? AO plus transparency can give some strange shadow artifacts, if I recall correctly.

Edit....  oh, never mind, if it's light-based AO it's ignored anyway.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)