[Zope] PersistentMapping not persistent?

[alex@quad.com.ar]

hey there,
I've been trying to get around a problem in my zope 2.4.2 persistentMapping.
I've built the zodb auth source of exIserFolder.. it basically uses a
PersistentMapping to store all the user accounts, passwords and roles.
Everything works fine, but then I noticed passwords getting reset when zope
restarts (or just on a long non-activity period)
I create a user, set his password and roles.. everything goes ok.. I change
his password to something different.. and it works.. but then I restart
zope, and his password gets resetted to the old one.
now if I use a
self.data={} instead of self.data=PersistentMapping() and then add a
self._p_changed=1 on every method that updates the self.data everything
works as expected, so here goes the question:

is PersistentMapping fscked up on 2.4.2 or does it still need something
similar to self._p_changed=1 to commit all the transactions back into zodb ?

any hints, I could fix this by NOT using PersistentMapping, but since that's
what User.py uses, I thought it was going to be a good way to do it....
please.. help :P

Alex