Hi @dangerous,
What is the condition of the brushes i.e. not worn, good tension on the commutator?
What is the condition of the commutator, segments clean etc?
How long does it take for the motor to stop when the power is disconnected?
If more than a few seconds then there might be a problem with the braking system on the motor, if it has one that is and this could cause the brushes to spark.
Here's a link that describes in general how the [https://www.protoolreviews.com/how-does-a-saws-electric-brake-work/|braking system] works.