If I'm understanding this correct then it sounds like a frequency setting problem?

Have a look at the first post here http://www.mycncuk.com/threads/5814-...nd-Advice-More and go through your setup again and make sure that the setings are as per the values stated, specifically PD011 (I set this to 0 so it can go all the way down to 0RPM though I don't know how useful this will be!!)