Universe Sandbox 2 keeps pausing at regular intervals?

Can anyone help me... I have a powerful computer but when i run universe sandbox 2 i get this regular pause in the framerate - as you can see the gpu seems to have nothing to do for a fraction of a second and the motion in the simulator stops then it starts again - it seems regular as clockwork - cpu is a 12 core ryzen 3900x and is no more than 30 or 40 % utilised during this video.
Any ideas how i debug the cause of this - not sure if i should be looking within the universe sandbox settings or at the PC as a whole
Any help appreciated