Old Walkman WM-FX12 sounds bad when shaking it

[Tijssen]

Hello everybody,
 
I have this old WM-FX12 Sony Walkman. It plays great when it is resting in its place. But when you take it in your hand and start moving it, it sound bad. It sound like it moving too slow and than too fast en like it going to stop (which) is not happening actually. Something with the belt? Anyone knows what the cause of this can be?
 
Thanks,
Luc