To examine the issues concerning Rydberg-state stabilization in pulsed-field ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy, spectra at the first and second ionization thresholds of argon have been taken under different ion-density conditions. These provide a contrast between the signals from autoionizing and nonautoionizing Rydberg series. A careful comparison between the PFI-ZEKE spectra and relative partial photoionization cross sections provides the first measurement of the efficiency of stabilization in PFI-ZEKE spectroscopy. This efficiency or quantum yield varies at the second ionization threshold with changing ion density and principal quantum number, introducing ion-density-dependent line-shape distortions. These changes may be ascribed to the competition between stabilization and autoionization of the Rydberg states and its variation with principal quantum number.