Sustained spin on a Bosch WFF 2001

I have had Bosch WFF2001 washing machines for more years than I care to remember and changed brushes, motors, shock absorbers, door catches etc, but the current issue is that after renewing the brushes for the second time in the machine’s life (after carefully inspecting and cleaning the commutator), the machine is reluctant to spin properly. Initially in the spin cycle, the motor and drum will turn smoothly, but at the point when it would normally accelerate to high speed, the motor seems to be ‘pulsing’ with a fair bit of sparking coming from the brushes (which have been changed again, with no improvement), never reaching a sustained high speed spin. I get the feeling that something might be a problem with the motor control circuitry, but does anyone have any experience of this behaviour and how to rectify it, please?
Many thanks.