  Context. Stars on the asymptotic giant branch (AGB) are long-period variables
that present strong flux variations at almost all wavelengths, including the
SiO maser lines. The periods of these variations are of 300-500 days in
Mira-type stars and somewhat shorter in semi-regular variables. The variability
of the SiO lines on short timescales has been investigated, but the data are
inconclusive. Aims. We aim to study the time evolution of the SiO maser lines
in Mira-type and semi-regular variables at short timescales. We also discuss
the origin of the observed fast variations. Methods. We observed the SiO maser
lines at 7 mm (28SiO v=1,2 J=1-0) and 3 mm (28SiO v=1 J=2-1) using the 40 m
Yebes antenna and the 30 m IRAM telescope, respectively, with a minimum spacing
of 1 day. We studied the semi-regular variables RX Boo and RT Vir and the
Mira-type variables U Her, R LMi, R Leo, and $\chi$ Cyg. We performed a
detailed statistical analysis of the variations on different timescales.
Results. RX Boo shows strong and fast variations in the intensity of the
different spectral features of the SiO lines at 7 mm and 3 mm. On a timescale
of one day, we find variations of >10% in 25% of the cases. Variations of
greater than $\sim$50% are often found when the observations are separated by 2
or 3 days. A similar variation rate of the SiO lines at 7 mm is found for RT
Vir, but the observations of this object are less complete. On the contrary,
the variations of the SiO maser line intensity in the Mira-type variables are
moderate, with typical variation rates around <10% in 7 days. This phenomenon
can be explained by the presence of particularly small maser-emitting clumps in
semi-regular variables, which would lead to a strong dependence of the
intensity on the density variations due to the passage of shocks.
