  There is new evidence which identifies seasonal changes of the variability
time scale in intraday variable sources with refractive interstellar
scintillation effects. Such a RISS-model takes the annual change of the Earth
velocity relative to the scattering medium into account. In September 1998 we
found a remarkable prolongation of the variability time scale in the IDV-source
0917+624 with only small variations in flux density during a period of 5 days.
This was explained as a seasonal effect, in which the velocity vector of the
Earth and the interstellar medium nearly canceled. In order to further
investigate the applicability of the model for 0917+624, we performed an
Effelsberg 6cm-flux monitoring program over the course of one year. Since
September 2000, the source appears to be remarkably inactive and yet (May
2001), no return to its normal, faster and stronger variability pattern is
observed. Here, our observational results and a possible explanation for the
current quiescence are presented.
