S600 Z axis problem

my S600 rehabilitation after assembly to operate in all axes X, Y and Z. Now There was a problem with the Z axis. When starting a job or work process becomes fast . I tried to change velospeed to 5000 and back 3000 but does not help.
Checks thread rod straight, pagriest behind the axle can be easily and rolls are not idle . Maybe someone knows what else can be done ?