Oscillation of elastic 1D-beam not constant

Hello,

My problem is that the oscillation of my elastic beam or every other elastic system is not constant. Every oscillation, the curve shifts. A Picture is attached! You can see it if you look for example on the wave peaks.
I need to do this with Abaqus/Explicit; the oscillation is triggered by an impulse.
I would be very happy about an answer.

With Best Regards,
Max