Abstract: High-power O-mode radio waves can excite artificial instabilities in the F region, according to experiments conducted at the European Incoherent Scatter Science Association (EISCAT) heating facility. The main instabilities include the parametric decay instability (PDI), oscillating two-stream instability (OTSI), and thermal parametric instability (TPI). The PDI and OTSI not only compete with each other, but also compete with the TPI, leading to a two-stage overshoot phenomenon: a miniovershoot occurs on a millisecond time scale after pump-on, followed by the main overshoot. We gain insight into the miniovershoot via a generalized Zakharov model, whereas the main overshoot can be observed as an enhanced plasma line overshoot phenomenon in incoherent scatter radar spectra. We can also observe that the zero-frequency ion line exists only in the initial heating period after a cold start and that the upshifted and downshifted ion lines behave irregularly in the spectra. The simulation results show that competition between the PDI and OTSI leads to an initial peak, which we named the pre-miniovershoot. The following processes, namely ion density caviton generation, and collapse and cascade in the development of the PDI, contribute to the miniovershoot phenomenon.