This is a long shot, but I have encountered 3.5" drives that were in very dirty industrial environments where lots of grinding/sanding/drilling of metal happened. And the fine metal debris that got sucked into the case found it's way into the direct drive motor due to the magnets.

Some photos would help. Common failure points of the spindle (other than too much dirt) are the ceramic resonator and electrolytic caps on that sub-PCB. If the resonator dies the motor won't even start but unstable frequency (which too can be caused by dirt or corrosion) will cause RPM deviation. The caps usually leak, and corrode nearby stuff, but sometimes also dry out without any external signs. It's often possible to tack a temporary cap on the back to see if it helps any, before commiting to desoldering.