@johnRob
Spot on. My motor was clearly overheated and the isolation in the rotor windings was toast, short circuited, inside. At the brushes and the collector there were sparcs like new years fireworks. The RPM of the motor was like twist and shake.
As I said, running the motor for a long time made the temperature curve point upwards without any sign of settling. That was measured on the new, spare part motor.
Heavy duty stuff is required for 10 - 12 hour continues running. Chinese hobby motors is the wrong choice.