Dear Barry,
The rapidly varying blade-pitch angles you are seeing in your Simulink model are likely caused by the extrapolation of inputs implemented within the FAST S-Function (as discussed in other forum topics e.g. S-Function input output signal mismatch - #2 by Jason.Jonkman). My guess is the pitch angle of 5 degrees that you are setting in your Simulink model does not match the initial blade-pitch angle set within the ElastoDyn primary input file, hence the large variation in pitch angles resulting from the extrapolation. I would always ensure that (1) the initial pitch angle set within the ElastoDyn primary input file matches the pitch specified within the Simulink model at time zero and (2) the pitch angle changes are defined smoothly within Simulink.
Best regards,