Ahh, a slow see-saw like bounce. THat's not dampers, that's most likely a servo going bad. Those s75s will fail in such a way as to lose their sense of "center" and they will wander a bit. I almost lost control of my MX400Pro on maiden hover because of one of those things doing that (they came with the DX6, I was just using them to set-up, never again will I make that mistake).


This diagnosis also explains the fact that it got worse when you _removed_ a shim, not better. THe heli is more responsive with tighter damping, so you are seeing it respond to the bad servo more.

Since the nod is in the pitch direction (nose up and down) I'll bet it's the front servo. If it were one of the others the motion would be a pitch-roll couple.

But first eliminate all binding and/or play in the links to that servo, before you go and replace it.
If you do need to replace a servo I recommend replacing all three with HS55s.