A few
somewhat easy things I would try:
1. Remove belt and platter and check on the level of fluid in bearing cup. With spindle in place, the fluid must be high enough to cover the top two contact points. (A flashlight is helpful.) If you've never changed the spindle oil, you might consider doing so. The easiest way is to use a syringe and some q-tips; it doesn't need to be perfect. Be careful not to disturb the setting of the contact points: two at top, two opposite at bottom, and one on the floor. Also, don't torque the bearing cup; it is aligned a specific way relative to the motor. For refill, standard motor oil can be used; search around for the proper rating, I think 5W20 or 5W30. Don't overfill or it will all gush out when you put the spindle in place. Start small and iterate until you've added enough oil to fully cover the top contacts.
2. While the platter is off, pull out the motor and check for anything that might be fouling its movement. Blow out any dust. Using a needle-tip oil applicator, apply a very small amount of oil to the shaft below the pulley and at any other place you can see within the motor assembly where it might be helpful. Don't overdo it! Plug the motor into power and let it run for 10 minutes before wiping off any excess oil and putting everything back together.
3. Keep all of these lubricants away from the belt and pulley! Before final assembly, clean pulley contact surface with q-tips and isopropyl alcohol. Wash hands before handling the belt.
With everything you might need on hand, this whole process should take under an hour. If the problem persists, it's likely not something you can fix yourself. Good luck!
P.S. I'm sure you know this, but make sure the platter is running level when powered on. The tension of the belt holds it upright when otherwise it has a tendency to flop over to the right. Also, make sure the motor spindle is perfectly perpendicular to the belt plane. When running, the belt should ride on the center of the pulley (see photo) and not rub on the flanges.
