Can lenboard cause light leak?

Hello, kind and experienced photographers, I am being driven mad by white lines on my negs, I was generosly given a lot of advice in pevious post which I made r egarding this problem. Here is a sample image :

http://www.duich.com/adriantylernet/site/white_lines.html

I have checked the bellows for leaks, checked the mounting of the bellows, used various filmholders and films, carefully shaded the filmholders at the time of e xposure until I am sure that no light is is leaking in throught the back of the contraption.

The latest results are completley free of leaks apart from the fine white line a round the image, could this be the lensboard? is it worth changing it, or should I give up and take it to a technician?

thanks again

adrian