install stable with testing installer cd

["MatB" <matteo.brusa@gmail.com>]

Hi,
I'm trying to install Sarge on a Dell SC1425 with scsi disks. Since the
"stable" netinstall cd does not work (problems with the aic79xx module)
i'm using the testing netinstall cd.
However, at the point of downgrading, the process hangs while replacing
the package passwd. Here's the error i get:

dpkg: error processing
/var/cache/apt/archives/passwd_1%3a4.0.3-31sarge5_i386.deb (--unpack):
 trying to overwrite `/usr/sbin/add-shell', which is also in package
debianutils

Any idea on how to fix this?
My other idea is to tell the installer to use in first place the stable
repository when  installing the system; is there any way to do it?

Thanks in advance, 

MatB