Composite modes & Alpha channels

Hi all,

Ran into a problem I was hoping someone could help me with. I made some groovy looking streaks using fractal motion that whizz from side to side of the screen to act as a background for some lower third text.

It looks great in AE, so I exported it for Final Cut using render settings RGB+Alpha and millions of colours+, thinking this would leave the streaks nice and bright and leave the black areas clear. But what happened was the black areas above and below the streaks were clear, but behind the streaks a nasty band of the black remained.

I tried playing with a few composite modes, but the only one that clears all the black is 'screen', and that leaves the streaks looking very washed out.

An interim solution has been to import the Final Cut edit into AE and put the streaks and text on there which it does perfectly, but of course I then lose the flexibility of being able to re-edit without taking everything back into AE again...

I've clearly missed a step somewhere, can anyone see where?

Thanks in advance