Once had flutter on aleron flutter on a DR400, it started at about 110kt and dispite reducing airspeed it would not stop.
As a last resort I stalled the aircraft and this stopped the flutter and then returned to the airfield at 70kt
The problem was the aleron cables had been rigged in mid winter in a very cold hangar with a high humidity, the inccident happend in mid summer during a period of low humidity.
The cause of the flutter was low aleron cable tension as the result of expantion of the steel cables due to the summer heat and the contraction of the wooden airframe due to the low humidity.