Does the platter exhibit any run out? If you get down at eye level and view the platter surface on a 90° angle can you perceive the platter surface undulating? Those might be signs that the little plastic buttons inside the bearing well that support the platter spindle shaft in the lateral plane might be wearing, perhaps due to a lack of or insufficient lubrication over a period of many years. As you probably know, in the well tempered design the belt is part of the vertical bearing mechanism. It pulls the platter shaft in one direction while those two white plastic buttons stabilize it in the other direction at a 180° angle To the belt. If you remove the belt the platter flops over toward the unsupported side in the bearing well.
