beaters lose power on high speed

Kenwood Chef A701A speed control and motor work, but beaters lose power as contol is turned from mid range to high. Have checked belt and it is okay. Perhaps central spindle on it is slipping past lugs or maybe it's something inside above where the beaters fit? Not sure how to get this apart - would have to take out central spindle where belt is attached and then undo screws I think. Not sure how to get spindle out (it has a spring on it) and would rather not do this without confirmation.