  Proto-planetary nebulae (pPN) and planetary nebulae (PN) seem to be formed by
interacting winds from asymptotic giant branch (AGB) stars. The observational
issue that most pPN are bipolar but most older PN are elliptical is addressed.
We present 2.5D hydrodynamical numerical simulations of episodic cooling
interacting winds to investigate the long term evolution of PN morphologies. We
track wind acceleration, decrease in mass-loss and episodic change in wind
geometry from spherical (AGB) to collimated (pPN) and back to spherical again
(PN). This outflow sequence is found to produce realistic PN dynamics and
morphological histories. Effects from different AGB distributions and jet duty
cycles are also investigated.
