  We have performed numerical simulations of a radially perturbed "accretion"
torus around a black hole or neutron star and find that the torus performs
radial and vertical motions at the appropriate epicyclic frequencies. We find
clear evidence that vertical motions are excited in a non-linear resonance when
the applied perturbation is periodic in time. The strongest resonant response
occurs when the frequency difference of the two oscillations is equal to
one-half the forcing frequency, precisely as recently observed in the accreting
pulsar, SAX J1808.4-3658, where the observed kHz QPO peak separation is half
the spin frequency of 401 Hz.
