oscillations in LS-Dyna acceleration-time output

I hope this is the correct forum to post this in ... 

I am new to Ls-dyna but as part of a research project I am modeling a point impact on a fixed-fixed beam in LS-Dyna (the impact is more than sufficient to fail the beam plastically).  However, in the acceleration-time plot I get increasingly severe oscillations when the D3plot output frequency is decreased. Looking at the images below, the first is for an output frequency of 0.01ms and the second is for an output frequency of 0.5ms (total simulation time = 16ms).  The second plot (0.5ms output frequency) matches experimental results well.  I have tried adding contact damping of 90% as well as hourglass type 4 coeff 0.03 but these had no effect. 

Does anyone have any suggestions about why this could be happening and how to correct it? 

Other information:

dt = 2.01263E-04
material model: MAT24 - piecewise linear elasticity
contact type: automatic single surface
beam is modeled using Belytschko-Tsay formulation shells (all quads) with 5 through thickness integration points
impacting mass is modeled as a rigid solid

Any help would be greatly appreciated.
Thank you in advance,



 D3plot frequency = 0.01ms



D3plot frequency = 0.5ms