Would it be possible to supply a swf that reproduces the issue, and source if you can manage it? You can email them to me directly at firstname.lastname@example.org. I will log a bug in the meantime to make sure we follow up on this issue.
I can also see this / a similar issue on Windows 7 / Firefox 10.0.2 / Flash player 11,0,1,152
The wite areas change as the swf udeates / renders... and it seems to be related to bitmaps that change (e.g. animate / have mouseover effects etc...)
We confirmed the issue reported here and it has been fixed in our next upcoming release. Appologies for any inconvienience. To verify if it is fixed you can try your content with our latest beta release found here:
Please let me know if the issue persists with that build.