conundrum: mixed-down sequence plays fine in Avid, out-of-sync when exported?

Hi there,

I've tried exporting a nine-minute sequence every way I can think of (QT, Ref., Same-as-source, for Squeeze, etc.)... and every time the file I'm left has audio that's out of sync.

Any suggestions re what I can try next?  I've got to get this thing exported and delivered...

Thanks,

Malcolm