It sounds like the motor is probably caked with crud where the filter hasn't been washed often enough. Same problem as the DC24 motor, they're only small and can't take much punishment. Let's be honest, who remembers to wash out the filters monthly as recommended!?
Problem is, you can clean out the motors to a certain degree but they never run the same again.
There isn't a help guide available yet for the DC50 unfortunately. I've just a motor change on one but it's pretty tough going!