I agree that it could be the belt. There are actually two belts in your Beogram but we are referring to the large one that drives the platter. New belts are in stock.
It could also be the speed setting trimmers. These are electronic parts inside the Beogram. Some trimmer types fall apart physically, others oxidate heavily. They are fairly cheap components but basic electronic skills and tools are required to replace them.
However, I would check the belt first. Lift off the platter (it can bind drastically to the subplatter on which the belt grips). With the Beogram disconnected from mains rotate the subplatter back and forth by hand, watch if the motor rotates or the belt slips. If the belt slips it will definitely need replacing.
Martin