here you go mate - using f9 is a plaster, not a fix, sounds like you are seeing the same i was and your issue may compound like mine did so keep your eyes on it…there is video in the thread below too, understand that when i first started seeing it, the issue was not so bad but then it got worse and worse until it got bat schit mental as you will see in the video, at times i have had sustain completely flatten itself :-o
sad to say that mine is boxed and in the next day or so will be on its way for repair