4th axis and fusion 360 problem

[amxen]

Having real problems with my 4th axis . Can any one help. IV just installed it and made a small model in fusion this is my first time using the 4th . When I run I code it runs ok for the most but the the 4th (A) slows down to almost a stop . It crawls along at about 1mm a minute . Then after it gets to where it's going all speeds up again. It does this 2 or 3 times in the operation . IV run it 5-6 times and it happens at the same place every time. I'm scratching my head not knowing if it's the post processing or fusion or my new 4th axis or my stepper. iv added some videoAny help would be cool