some vfd has this
I remember my first spindle made it..
and I don't even could set to 400 hz
best you can do, use it as long as you can, and change it when you get chance.. I had a sunfair c300 vfd.. if it helps..
I wrote email to manufacturer , they answered.. but fluctuating were remain..
changing both, spindle and vfd I didn't experienced anymore..
I look around on other computers, because I remember the manual was discussing the fluctuation.. with settings you can reduce it..