The elastic pendulum is a 2-degree-of-freedom, nonlinear device in which the pendulum bob may slide up and down the pendulum arm subject to the restoring force of a linear spring. In this study, radial motion (motion along the arm) is excited directly. Responses to this excitation include purely radial oscillations as well as swinging motion due to a 2:1 internal resonance. Changes in the behavior of the nonlinear spring pendulum occur when, under the control of a parameter, radial oscillations become unstable and are replaced by radial plus swinging motion. This bifurcation is explored analytically, numerically and experimentally, using the basic ideas of Floquet theory. Poincaré sampling is used to reduce the problem of describing the stability of a limit cycle to the easier task of defining the stability of the fixed point of a Poincaré map.