I would suspect worn dampers, slider, loose bearings.... might be a good time to check ball links, sliders, any part of the control from the servo gears to the blade grips.... anything that is loose can sit fine for a while then start to flap around like an old shopping cart wheel. Just grab the paddles and see if there is free play when you twist it lightly, it is OK if it flexes a bit, but should be relatively tight.