Intense plasma waves are generated by an HF pump wave in an ionospheric heating experiment at the Arecibo Observatory. These plasma waves can be observed as enhancements to the ion and plasma lines of the incoherent backscatter echo. The enhancements can be 3 or 4 of magnitude more intense than the unehanced lines and tend to fluctuate wildly. Both the purely growing and the decay mode parametric instabilities are present. When the pump wave is turned on abruptly, the enhancements develop in time in a repeatable manner. A rather remarkable feature on time scales of seconds is an over-shoot in instability power. Overshoots occur frequently but not universally and last for 1--6 s. They can have a magnitude from 10 to hundreds of times the average instability level. The growth of field-aligned irregularities may be the cause of the overshoots. The overshoots appear to be definitely related to an unusually rapid rise in measured electron temperature that cannot be understood in terms of ohmic energy deposition. On time scales of milliseconds there is a 'miniovershoot' before the growth of the instability to a large value. The spectral details also change in a striking manner. The instabilities can first be detected 2--4 ms after pump wave turn-on. The decay mode is present as well as a broad featureless 'noise bump' which partially sharpens into a line as time progresses. These changes of the spectra in time seem to run counter to the currently accepted theories of plasma wave saturation.