  We present a detailed analysis of 30 very long baseline interferometric
observations of the BL Lac object S5 1803+784 (z=0.679), obtained between mean
observational time 1994.67 and 2012.91 at observational frequency 15 GHz. The
long-term behaviour of the jet ridge line reveals the jet experiences an
oscillatory motion superposed on its helical jet kinematics on a time-scale of
about 6 years. The excess variance of the positional variability indicates the
jet components being farther from the VLBI core have larger amplitude in their
position variations. The fractional variability amplitude shows slight changes
in 3-year bins of the component's position. The temporal variability in the
Doppler boosting of the ridge line results in jet regions behaving as flaring
"radio lanterns". We offer a qualitative scenario leading to the oscillation of
the jet ridge line, that utilizes the orbital motion of the jet emitter black
hole due to a binary black hole companion. A correlation analysis implies
composite origin of the flux variability of the jet components, emerging due to
possibly both the evolving jet-structure and its intrinsic variability.
