Hi Gunnar
My question is if the slowing down is sometimes for a second or slowing down for longer time.
first case from my experience there is a problem with the spark snubber diodes (TVS diodes) in the motor. Find out detailed information here:
https://beolover.blogspot.com/2021/12/Beogram-4002-dc-motor-spark-snubber-replacement.html
second case from my experience are the dryed out motor bearings. Detailed information here:
https://beolover.blogspot.com/2023/11/beogram-4002-dc-platter-motor-restoration.html
and here:
https://beolover.blogspot.com/2021/10/beoparts-dc-motor-bearings-tested.html
Kind regards
Christian