Downgrading a system to a particular distribution

[Alf <alf@lanza-ti.com>]

Hi list,

I started off from a stable Debian, and got to upgrade some packages
like evolution (and its dependencies) to testing distribution (etch).
Also, in order to install some packages that were only in unstable, i
had to install some dependencies from unstable branch.

Now my system is no very stable, and some applications crash now and
then. Because of this, i'd like to return to a more stable state. Is
there a way to uninstall all packages from unstable branch? I'd like to
get my system back to testing, say, or stable.

Any ideas would be appreciated.

TIA