I bought a sanou lathe chuck which eliminated the shimmyI will take the chuck off and try it. I have always avoided that rpm range ant it works fine. What make of aftermarket chuck would be the choice to replace the factory chuck to aluminate the shudder? This lathe is about 4 years old.
What is a VFD?
if you find it is the chuck there are ways to balance it search the web I never tried it myself
vfd is variable frequency drive which gives motor speed control (variable speed )