check the accelerator switch and see if it has a high speed switch in it, this switch controls the 1A speed so it can trigger the truck to use 1A speed when the pedal is pressed down fully so it will not be activated by the 80% on-time timer in the scr signal.
it could be the switch is pulling in prematurely causing it to jump into high speed.
Of course it could also be the 1A timer is set too short in the settings.