  It is known that for some time periodic potentials $q(t, x) \geq 0$ having
compact support with respect to $x$ some solutions of the Cauchy problem for
the wave equation $\partial_t^2 u - \Delta_x u + q(t,x)u = 0$ have
exponentially increasing energy as $t \to \infty$. We show that if one adds a
nonlinear defocusing interaction $|u|^ru, 2\leq r < 4,$ then the solution of
the nonlinear wave equation exists for all $t \in {\mathbb R}$ and its energy
is polynomially bounded as $t \to \infty$ for every choice of $q$. Moreover, we
prove that the zero solution of the nonlinear wave equation is instable if the
corresponding linear equation has the property mentioned above.
