Motor runs OK speed control works OK, until the motor is put under increased load and it cannot cope, checked motor brushes and they are good and long. Any idea please
There are two areas in the machine where sparking happens and each might reduce the motor's efficiency which might be revealed under heavy load conditions.
The speed control itself - relatively easy to service - just clean up the contacts and remove the soot.
Secondly and a bit more challenging is to clean up the motor's commutator until it has no blackening streaks on it - very fine abrasive paper while you rotate the armature by hand.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix blenders/mixers.