That link doesnt work.
kchriste is asking if your speed input PWM is smoothed to DC with a filter. Low PWM frequencies may pulse the motor.
I yahoo translated part of the manual (this is a laugh, make of it what you can ;-). It seems there is an acceleration setting pot (VR3), and either a current limit or IR comp pot (VR1), you should try and adjust these too. VR4 looks like a feedback setting of some sort that may affect stability.
CN1 AC inputs CONNECTOR CN2 CONNECTOR where is connected with MOTOR CN3 CONNECTOR where I/O is connected VR1 With electric current restrictive variable resistance the CCW direction becomes SETTING with 200% of orthodox electric current, Is and according to use use turns the possibility of raising an electric current price is but MOTOR and DRIVER the increase which is excessive the small hand the possibility of doing is the user getting a manufacture resignation technical inquiry, wishes with CW directions and VR2 To turn with CW directions with MOTOR maximum RPM sets and RPM to increase, CCW Turns with a direction and RPM decreases SETTING with correct rules RPM of the factory shipping at the time of MOTOR which becomes, being shipped, VR3 This DRIVER MOTOR RPM rate control volume the outside and speed inside The variable the possibility of doing in order to be at control volume, is planned and VR3 CCW direction decreases at internal rate control volume and CW directions increase. This time, the external volume use at the time of JP1 must change JP1 and JP2 JUMPER from the dignity layout sketch and JP2 1 time and time 2 JUMPER, internal volume use at the time of JP1 and JP2 2 times and 3 times JUMPER year. VR4 CW directions adding and subtracting inside do shortly with MOTOR adding and subtracting inside controls, CCW direction In order to be regulated an adding and subtracting inside long, is planned