Bitmap images in Symbols change their position (AI 10)

I've created several symbols that include embedded bitmap images. When I close and reopen the file, some of the bitmap images move several inches down relative to the rest of the objects in the symbol. Because the bitmap is used in several different symbols I have used a "symbol" of the bitmap instead of duplicating the bitmap in each symbol. Could this be part of my problem? The program allows me to "nest" symbols, so I don't understand why the bitmap is changing position. Any insights?

Oh! Another point is that these symbols are being distorted with a distortion mesh. Interestingly, if I break link to the symbol, then release the distortion envelope and emediately reinstate the distortion, the bitmap in question pops back in place. I know this is confusing. Just hope someone can relate to the problem.

Thanks,
Ray