  Short time-scale radio variations of compact extragalactic radio quasars and
blazars known as IntraDay Variability (IDV) can be explained in at least some
sources as a propagation effect; the variations are interpreted as
scintillation of radio waves in the turbulent interstellar medium of the Milky
Way. One of the most convincing observational arguments in favor of a
propagation-induced variability scenario is the observed annual modulation in
the characteristic time scale of the variation due to the Earth's orbital
motion. So far there are only two sources known with a well-constrained
seasonal cycle. Annual modulation has been proposed for a few other less
well-documented objects. However, for some other IDV sources source-intrinsic
structural variations which cause drastic changes in the variability time scale
were also suggested. J1128+592 is a recently discovered, highly variable IDV
source. Previous, densely time-sampled flux-density measurements with the
Effelsberg 100-m radio telescope (Germany) and the Urumqi 25-m radio telescope
(China), strongly indicate an annual modulation of the time scale. The most
recent 4 observations in 2006/7, however, do not fit well to the annual
modulation model proposed before. In this paper, we investigate a possible
explanation of this discrepancy.
