Masking transition glitch

Hi all, slight issue with a flash banner I’m working on.

You can see it here: http://www.greenmountaindesign.co.uk/march.swf

As you will notice, when the square mask thing comes in (first seen with the blue mustang) a line on the right hand side can be seen, which remains for the duration of the slideshow - so its obviously an issue with the mask.

I have tried tweaking it, but can’t seem to resolve the issue.

Any advice?

Thanks for reading

Andy