  We present magnetohydrodynamic (MHD) simulations of the evolution from
quasi-equilibrium to eruption of a prominence-forming twisted coronal flux rope
under a coronal streamer. We have compared the cases with and without the
formation of prominence condensations, and the case where prominence
condensations form but we artificially initiate the draining of the prominence.
We find that the prominence weight has a significant effect on the stability of
the flux rope, and can significantly increase the loss-of-equilibrium height.
The flux rope can be made to erupt earlier by initiating draining of the
prominence mass. We have also performed a simulation where large amplitude
longitudinal oscillations of the prominence are excited during the quasi-static
phase. We find that the gravity force along the magnetic field lines is the
major restoring force for the oscillations, in accordance with the ``pendulum
model'', although the oscillation periods are higher (by about 10% to 40%) than
estimated from the model because of the dynamic deformation of the field line
dips during the oscillations. The oscillation period is also found to be
slightly smaller for the lower part of the prominence in the deeper dips
compared to the upper part in the shallower dips. The oscillations are quickly
damped out after about 2-3 periods and are followed by prominence draining and
the eventual eruption of the prominence. However we do not find a significant
enhancement of the prominence draining and earlier onset of eruption with the
excitation of the prominence oscillations compared to the case without.
