GAL loses accounts

we have a mixed mode domain, and mixed mode Exchange, with both 5.5 and 2003 servers.

We randomly lose accounts that are hosted on the 5.5 when viewed from a client who is hosted on either 2003 or 5.5.

Moving the user to 2003, and back again to the 5.5 seems to get them back on the GAL, but what is the cause of this random loss ?