Trackspot mirror is not running smoothly

When the mirror is moving, it not running smoothly, not in fast or in slow mode. It's like that there are send pulses and pauzes to the motor rather then only pulses. (at the moment I don't have a scope :()
When another motor is used, it has the same effect. The mirror is not blocked in any way, when moved by hand you don't feel any obstruction.
I have already tried other motor drivers. Gobo and Color wheels are running without any problem.
Perhaps there someone who recognize this 'problem' and knows the solution for this effect.
Staf
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