HELP: Volume Effects Shader ( XSI 2012 )

[Krallis]

Im at the end of my tether here.
This damn shader will be the death of me.
Im using it to create the effect of Cherenkov radiation in my scene.
Its applied to a sphere surrounding my scene and this sphere also has a transparent surface material applied.

Now. It has never rendered reliably at all.
Sometimes it renders find and looks great. Then I could do the exact same render and change nothing. And it totally changed the result.

Two things happen. Either it renders fine and looks great. Or it turns into this super bright fog and you cant see anything through it at all.
Ive tried fiddling with a LOT of things to get it to work. The only things that seems to be a barrier is the geometry. If it renders bright turquoise I can just hid all the geometry and the fog effect renders fine. What I dont understand is why the fog effect sometimes renders perfectly fine with the geo and sometimes not.

This is what I dont get, surely theres a way to get it to render properly with all the geo visible

At the moment im experimenting with compositing and render passes because if the geometry is what's getting in the way then this may be the only way to reliably render it out.

This has just been a nighmare the last few days trying to narrow down whats causing this. And everytime i think ive got it nailed it always renders badly again. Im just at the end of my tether here and wondered if anyone here could help as I honestly do not know whats going on.

Ive attached images of the two ways it renders out.
(please ignore the text, it was when I was just trying everything to troubleshoot this)

And ive also attached the XSI user guide page for this shader, not that its been very helpful for me in solving this matter.

Any help anyone can give would be most appreciated.
Thank you.

[Krallis]

Ok ive found a compositing solution to get this to work.
But im still curious as to why these render issues happened in the first place.
If it was something that couldve been solved to render it in just one pass instead of using 2 composited together.

[Krallis]

Yeh, its still nto working, it rendered fine for a while but now its back to rendering the bright turquoise fog.