  We present an internal shock model with external characteristics for
explaining the complicated light curves of gamma-ray bursts. Shocks produce
gamma-rays in the interaction between a precessing beam of relativistic
particles and the interstellar medium. Each time the particle beam passes the
same line of sight with the observer the inter stellar medium is pushed
outward. Subsequent interactions between the medium and the beam are delayed by
the extra distance to be traveled for the particles before the shock can form.
This results in a natural retardation and leads to an intrinsic asymmetry in
the produced light curves for gamma-ray bursts. In addition we account for the
cooling of the electron-proton plasma in the shocked region, which gives rise
to an exponential decay in the gamma-ray flux. The combination of these effects
and the precessing jet of ultra relativistic particles produces light curves
which can be directly compared with observed gamma-ray burst light curves. We
illustrate the model by fitting a number of observed gamma-ray bursts which are
hard to explain with only a precessing jet. With a genetic algorithm we are
able to fit several observed gamma-ray bursts with remarkable accuracy. We find
that for different bursts the observed fluence, assuming isotropic emission,
easily varies over four orders of magnitude from the energy generated
intrinsically.
