  We study modulational instability in a dispersion-managed system where the
sign of the group-velocity dispersion is changed at uniformly distributed
random distances around a reference length. An analytical technique is
presented to estimate the instability gain from the linearized nonlinear
Schr{\"o}dinger equation, which is also solved numerically. The comparison of
numerical and analytical results confirms the validity of our approach.
Modulational instability of purely stochastic origin appears. A competition
between instability bands of periodic and stochastic origin is also discussed.
We find an instability gain comparable to the conventional values found in a
homogeneous anomalous dispersion fiber.
