Yeah, it didn’t sound that bad in the videos, at least on my iPad, and if you can’t reach top speed any longer, then it does need to be solved. Are you saying at full speed it is now 500 rpm less than before, or you can’t run it at full speed because of the vibration?
If you can’t reach full speed any longer, then it could be a drive problem. A drive problem can cause a torsional vibration problem that would give you symptoms like you are seeing. I would still check the following to make sure there are not any mechanical issues.
Take the belt off and spin the spindle by hand, does it spin smoothly without any indication of chatter or pulsating resistance? If the bearings are not seated squarely to the shoulders, you could be getting binding during the rotation, but since you said the surface finish is good, I don’t think this is it, but should be checked to eliminate it as a possibility.
Measure the runout of the spindle pulley and motor pulley in the grooves, as well as axially. If either do not look good, take the pulley off and see if it is the pulley, or the shaft. You want to measure in the grooves since that is where the belts are running.
If you run the motor without the belt on, does it give any indication it is pulsating? Since you mentioned earlier that it doesn’t do this on the low speed setting, but does on the high speed, then I would think this is probably ok, but should be checked anyway.
I wish you lived nearby, troubleshooting is much easier in person than 3,000 miles away.
