SDS speed control

Have you changed, or applied talc to your belt & lubed the motor lately? Cleaned the circumference of the platter and pulley, with alcohol? If your belt is slipping significantly; no amount of correction from the SDS will help.

What strobe disc and light are you using? The VPI supplied strobe disc is pretty much useless, and I didn't have much more luck with a decent quality Hansen strobe disc and a "neon" light until I got the KAB Speed Strobe and used the quartz illuminator to determine the platter speed. You really need to be able to see the strobe disc markers clearly, which could be difficult without proper light.

If your belt is slipping significantly; no amount of correction from the SDS will help.

I would think something that severe would be immediately audible....

Shakey

How does the table sound? If the table sounds alright, then perhaps you are using the strobe disc and light incorrectly. What are you using as a light source and strobe disc?

if your wooden/plastic motor spindle the belt touches is the wrong diameter the sds will be adjusted to a level it is not designed for and never settle down. this happened to me. vpi sent me a smaller diameter and cured problem