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That sort of behavior is abnormal.
Now, in these treadmills, a speed sensor error often disables the treadmill altogether. This happens when the rotation speed of a rotating part inside the treadmill is not sensed by the circuitry (for one of several reasons.) So, it stops, and shows an error code.
But, if the speed sensor is simply marginal in performance, and misses a few rotations now and then, the speed will be sensed briefly as too low and the treadmill may speed up briefly in response - and then the speed sensor being fully back online and working, it will sense the speed is too fast and slow down again.
So the speed sensor being somewhat near being inoperative, but not quite, could cause the problem you have asked about.
Or, it could mean you have a defective, erratic, motor controller board.
So I think what is needed is, to have the 'mill opened up, and the speed sensor's alignment examined. It needs to be aligned (if it is the magnetic kind of sensor switch, that closes when it is near a magnet on the spinning portion of the machinery) so that it is merely a credit card's thickness away from the magnet on each revolution of the parts of the 'mill. If slightly farther away, if the gap is greater, the problem you ask about could happen.
Then if that is adjusted but it still behaves erratically, the motor controller board being replaced would need to be tried.