  We suggest that extragalactic radio sources are intermittent on timescales of
10^4-10^5 yr. Using a simple spherical model of a cocoon/shock system, it is
found that inactive sources fade rapidly in radio luminosity but the shock in
the ambient medium continues to expand supersonically, thereby keeping the
whole source structure intact during the inactive phases. The fading of
inactive sources, and the effect of the intermittency on the expansion
velocity, can readily explain the observed over-abundance of small radio
sources. In particular, the plateau in the observed distribution of sizes found
by O'Dea & Baum (1997) can be interpreted as being due to intermittency. The
model predicts that very young sources will be particularly radio luminous,
once the effects of absorption have been accounted for. Furthermore, it
predicts the existence of a significant number of faint `coasting' sources.
These might be detectable in deep, low-frequency radio maps, or via the X-ray
and optical emission line properties of the shock front.
