  We consider a linear differential system of Mathieu equations with periodic
coefficients over periodic closed orbits and we prove that, arbitrarily close
to this system, there is a linear differential system of Hamiltonian damped
Mathieu equations with periodic coefficients over periodic closed orbits such
that, all but a finite number of closed periodic coefficients, have unstable
solutions. The perturbations will be peformed in the periodic coefficients.
