I would say it is one of two things.
1. The as3x reciever is very sensitive when setting it up, you may need to redo the setup process, (i used a vid by horizon hobby on youtube) because it may be caused by a small amount of vibration.
2. A malfunction in the electronics, if you have a servo tester, you should test the servos to ensure it is not an issue with them, if not then it is most likely the as3x reciever.
(if it flutters more on high rates, then its the reciever for sure)