I am not familiar with Sony turntables, but have come across this same problem with Technics direct drive turntables. It is definitely not an issue with the motor windings. Also this issue has nothing to do with friction or lubrication. In Technics DDs, this occurs when the motor looses is torque. What I have done to resolve this problem is to start with the power supply and check whether the voltages before and after the rectifier and correct as per service manual specs and then focus on the regulator and filter capacitors. If the filter capacitors in the power supply or the regulators are faulty, you may encounter loss of torque. Also run a check on the cable connectors and contacts that link PCBs, and ensure that everything is in order.
The next set of suspects would be the electrolytic capacitors on the main PCB. You will need a capacitance meter and an ESR meter to check these. It is important that the voltages at the IC pins measure up (be very careful while doing this, especially with short circuits caused by the multimeter probes slipping). If the pin voltages don't measure up to the specs in the manual, immediately check the capacitors around in that circuit. If the capacitors are fine, there is a good possibility that you may require an IC change (this is the last step usually if all the other steps mentioned above throw up positive outcomes). I common way for folk to resolve this issue is to do a mass-electrolytic cap replacement. However I am not an advocate of this approach. The quality of OEM capacitors in these Japanese turntables is usually very good and hence do not replace them if they throw up good readings on capacitance and ESR. In Technics turntables, you would find electrolytics with red colour lettering. These are supposed to be of the highest quality and highly critical components. They should be replaced only with high quality capacitors with similar specifications and only if required.
