Does this aircraft have dual control capability? Does it happen with the copilot seat occupied? Do you feel uncommanded movement in your cyclic? I have heard of aircraft with the dual control provisions having the copilot cyclic cover plastic piece missing or loose enough that a person riding in the co-pilot seat can inadvertently kick or nudge the cyclic socket where the copilot controls would be installed causing some control movement that can scare the PIC. Just throwing stuff out there.