If it runs normally for a short time and then begins arcing and slowing down, that indicates that one of the carbon brushes is sticking...one of the less serious and more easily remedied issues that a motor can have.
Try taking it apart and replacing (or just removing, cleaning, and reinstalling) the carbon brushes.
My Thoro-Matic commercial canister that I use on service calls was modified many years ago with a motor from a Filter Queen and it has done the same thing to a greater or lesser extent for years. It's been fine lately and I never even got around to servicing it, but there have been times when I thought the motor was on its way out.
On the other hand, if it runs slowly and arcs at the brushes continuously, the motor is permanently damaged and needs to be replaced (unless you want to try your hand at re-winding the motor...)