Variability is a characteristic feature of most AGB stars. Unfortunately, spectroscopic time series covering many of the relevant molecular bands and the dust emission simultaneously do not exist. Because of the different sensitivity of various observable features to different parameters and components of the atmosphere/ envelope of an AGB variable (LPV) and because of the strong and time dependent interplay of pulsation, dust and molecules, such simultaneous and homogeneous data are the only way for a complete understanding of the physical processes involved. From the theoretical side, many of the tools for a quantitative interpretation of such complete spectroscopic time series are available now. The SWS on board of ISO covers the wavelength region where continuous dust emission starts to dominate and most of the important features in the spectra of LPVs (molecular bands of CO, SiO, H2O, HCN, C2H2 and C3, the solid state features of Silicates and SiC). While some of the mentioned bands and features can be observed from the ground, this requires different instruments and often excellent observing conditions. Other bands accesible with ISO are in spectral regions where the Earth's atmosphere is completely opaque. Only ISO can provide, probably for a long time, complete, homogeneous and simultaneous data of the molecular features and the dust emission in the atmospheres and envelopes of LPVs. Such data will be a unique opportunity to test and improve the time dependent models. We thus propose periodic SWS observations of a few selected 'typical' objects.