Read this thread. It is all about the spindle speed issues and Syil responses.
http://www.cnczone.com/forums/showthread.php?t=58937
Posts, in particular #10 and #13 give details of the technical problem as I see them.
I have offered to Syil that I will fix the problem for FREE. If enough people haunt them they might get something done.
Now that the X4+ is out they say the SX3 is no longer current, so it will never get fixed.
Make lots of loud noises, or keep using manual mode. Set PWM to about 600 is about as good as it gets. When I use auto mode, I let it settle using M0 to wait, then hit cycle start and walk away.
Don't use below 700 or above 3500 in auto mode.
On the X4+ don't use above 3200. You don't have much choice on the X4+
If its manual the axis is too. It has firmware issues reading the PWM / front panel knob properly.