Using exact matrix elements for the coupling, the effect of the anisotropy of the core on high molecular Rydberg states is studied by quantum dynamics. It is found that on the time scale which can be probed by zero kinetic energy spectroscopy there is extensive interseries mixing. In particular, the long decay times are due to the sojourn in Rydberg series which are not directly effectively coupled to the continuum. These are series built on higher rotationally excited states of the core and a dynamical bottleneck controls the coupling to the bound series directly coupled to the ionization continuum. The computations are carried out for realistic molecular parameters and in the presence of a weak external de field. The quadrupolar coupling is often more effective in interseries coupling than the dipolar anisotropy even though the latter has a far higher range. The external field exhibits the expected "dilution" or "time stretching" effect at short times (of the order of the Stark period) but enhances the interseries mixing at longer times. An incomplete I mixing is the origin of another dynamical bottleneck. The time evolution is described both by exact quantum propagation and by a reduced description where degenerate states (i.e., states which differ only in the magnetic quantum numbers) are taken to be equally populated, on the average. This grouping, valid at longer times, facilitates the quantal computations which include several series with the full complement of angular momentum states of the electron. Such computations are possible by taking advantage of the conservation of the (total projection) quantum number M. For higher values of IM the coupling to the continuum is very much hindered and the bound Rydberg series exhibit extreme stability. The paper concludes by an analysis of the three bottlenecks which can give rise to longer decays.