Fisher & paykel smartdrive 051 stops wash and/or rinse cycles/agitation and continually beeps. have to turn off the power button to restart but often cuts out again after a minute or three. runs through the spin cycle ok. possibly control board? i'd suspect if it was balance switch/toggle then it would cut out on the spin cycle also... many thanks in advance

You have somewhat confirmed the suspected part- the control board - but I think we need to confirm on this.
So make sure that the motor, the drain valve/pump , the inlet valves, are all working OK which means no power drain , no heating or malfunction.
Check the motor in particular if it is shutting off or is consuming or of low speed. Now also check the water level control switch .
If all seems OK check all terminals, contacts, joints and the PCB.
If so the program controller must be faulty.if possible swap a board to finalise.