Dear @Andre.White,
At certain yaw errors in the idling condition, a known blade-edgewise instability exists, e.g., as discussed in the following forum topic: Designing for yaw errors using FAST - #7 by Jason.Jonkman.
Note that in the parked/idling condition, I would expect that you’d set the initial rotor speed in ElastoDyn to zero (RotSpeed = 0).
Best regards,