true , I often, get this pb as well. It looks like the structure ( mesh?) is resonnating.
I've tried to refine the mesh , but the pb was still there , so I have no idea on how to solve this. I have been told the V11 HFSS version solves this?
you will get ripples in S parameters whenever your simulation finishes before the energy in the computational domain decays sufficiently?
so increase the accuracy to around -50db or -60db for enerygy to decay sufficiently at the time ur simulation finishes. this will remove the ripples in S parameters