We present the X-ray images of all the available Chandra observations of the galactic jet source SS 433. We have inspected the evolution of the arcsec X-ray jets, which have been recently found to be manifestations of in situ reheating of the relativistic gas downstream in the jets. The Chandra images reveal that the arcsec X-ray jets are not a long term structure; the structure varies, indicating that the reheating processes have no preference for a particular precession phase or distance from the binary core. Three observations made within about five days in May 2000, and a 60 ks observation made in July 2003 show that the variability can be very rapid, from timescales of days to hours. Moreover, the three May 2000 images show two resolved knots in the east jet getting brighter one after the other, suggesting that a common phenomenon might be at the origin of the sequential reheatings of the knots. We propose a model to interpret these brightenings in terms of a propagating shock wave, revealing a second, faster outflow in the jet.