Numerical stability analyses of envelopes of post-asymptotic giant branch stars show instabilities towards radial pulsations over a broad range of effective temperatures, although only stars with sufficiently high luminosity-to-mass ratios and high enough iron abundances are excited. Particular choices for the concentrations of the remaining chemical elements in the stellar envelopes appear to be of minor importance for destabilization. The instabilities turn out to be generated or at least strongly influenced by the enhanced iron bump apparent in the OPAL opacity tables. The pulsations of the central star of the young planetary nebula IC 418 can be modelled by adopting stellar parameters constrained by observations