Simple FEA in MATLAB/Octave - beam elements

I have been working on a simple model for a project at work on my own time. It seems to be more difficult than I originally thought but I was hoping someone might take interest and weigh in.

I have attached a simple model developed for use with Octave of a simple structure. The physical part consists of a corrugated plastic spring and a steel cable. I have tried to model the plastic spring with beam elements and the cable as a rod free to rotate at each end (assuming the cable is not slack this seems reasonable). It seems to me the significant displacements of some nodes might be causing my problems, in other words I think I might be dealing with a large deformation type problem and my model may (unknown to me initially) rely on the assumption of small displacements.

The loads in the cable are much too large and some of the plastic entities deform excessively in the current model.

Any advice would be appreciated,
- MJB