  We consider an O(N) model coupled self-consistently to gravity in the
semiclassical approximation, where the field is subject to `new inflation' type
initial conditions. We study the dynamics self-consistently and
non-perturbatively with non-equilibrium field theory methods in the large N
limit. We find that spinodal instabilities drive the growth of
non-perturbatively large quantum fluctuations which shut off the inflationary
growth of the scale factor. We find that a very specific combination of these
large fluctuations plus the inflaton zero mode assemble into a new effective
field. This new field behaves classically and it is the object which actually
rolls down. We show how this reinterpretation saves the standard picture of how
metric perturbations are generated during inflation and that the spinodal
growth of fluctuations dominates the time dependence of the Bardeen variable
for superhorizon modes during inflation. We compute the amplitude and index for
the spectrum of scalar density and tensor perturbations and argue that in all
models of this type the spinodal instabilities are responsible for a `red'
spectrum of primordial scalar density perturbations. A criterion for the
validity of these models is provided and contact with the reconstruction
program is established validating some of the results within a non-perturbative
framework. The decoherence aspects and the quantum to classical transition
through inflation are studied in detail by following the full evolution of the
density matrix and relating the classicality of cosmological perturbations to
that of long-wavelength matter fluctuations.
