You have to remove the bottom cover in order to get to the two big screws that hold the side plate and motor on. I am away from my Bernina 830 right now, but I do remember you have to take off that bottom plate, then remove screws to take off the side panel. Then you remove the motor and slip the pulley off the belt. It's a good idea to check the motor brushes and see what they look like.
If I'm not mistaken, the foot controller is also the speed controller, and I think it's normal for the machine to go faster or slower based on how much you depress the foot control. So what you are experiencing with speed may be normal.
When I get home to my Bernina in a few days I can check it out more for you. I have that exact machine.