"jerking" motors when turning LCD Knob

Hello RepRappers

I have the following symptom....
when I turn the LCD Knob to adjust e.g. the spped multiplier the motors satrting to jerk.
There are definitely no lost steps....
It is more like an data jam.....

my configuration is an siple XY Printer
Elektronic RUMBA with 32 Steps Pololus (16 Steps is identical)
Firmware Repetier orMarlin (makes no difference)
Full Graphics LCD

any clue out there ?

thanks and best regards
Gerhard

We might be having the same problem, I posted a topic on this as well. Whenever my motors move, they move in sections, instead of point A to B. Are we having the same issue?

I wouldn't mind solving this cause I can't print anything