Excel documents with samba

Hello,

Samba 3.0.25b

I have a big problem on my samba fileserver and i don't know to resolve it !

A user open excel document
A second user open the same document
This second user sometimes can open the files in RW mode !!!
Or the system lost user rights "rwx rwx ---" becomes "r-- rwx ---"

I don't understand what happen,

Please help !
Thanks,
Alain