Extruder Stepper Motor stop working

[merlin01]

Good Morning.
I've a DIY Prusa i3 Aluminium with sanguinololu 1.3a and firmware marlin 1.0. Very nice printer until the extruder stepper motor started to malfunction: the filament is not pushed, the motor constantly takes one step forward and one step back when required, instead of moving forward. This problem occurs during printing and through the menu from the LCD.
Attempted solutions:
swap the drivers with the other axes;
buy and replace with new extruder;
replace the wiring;
replace the original sanguinololu with a new same version.
Results:
none.
Moreover, with the new board the movement on the z axis is reversed, the limit switch is not recognized with the risk of damaging the stepper motors which continue to force as they are not controlled by the limit switch.
I'm a bit confused and discouraged