It could be something with the motor itself, the motor controller, or a capacitor - if you have access to the control board in it, see if any capacitors may be inflated.
That said, I had unusual speed and heat issues with my 10+ year old unit and discovered the outlet and/or circuit was problematic. My unit worked great once I plugged it in elsewhere on a different circuit (i.e. routed to a different breaker). i.e. try a different outlet that is separate from the one you typically use?
Of course, the suggestion to send it in to Bamix may be good, too 
I probably would do that with mine since I have a long-shafted one.