  Comprehensive VLBI and multi-waveband monitoring indicate that a single
superluminal knot can cause a number of gamma-ray flares at different
locations. However, the often very rapid variability timescale is a challenge
to theoretical models when a given flare (perhaps the majority of those
observed) is inferred from observations to lie near the 43 GHz core, parsecs
from the central engine. We present some relevant observational results, using
the BL Lac object AO 0235+164 as an example. We propose a turbulent cell model
leading to a frequency-dependent filling factor of the emission region. This
feature of the model can provide a solution to the timescale dilemma and other
characteristics of blazar emission.
