Mahmood : I supose we should ask what fails on the controllers and what voltage they operate on . Are the dc motors permanant magnet or wound stator . If they are not permanant magnet design are they series field , compounding ect.
Finally how are the brushes and springs. is the commutator running true and are the brushes phase adjustable, if so are they set correctly?

In my experience a lot of the associated electronics fails from brush related causes. What happens is that the brushes start to bounce, or stick in the holders. Either fault causes a loss of connection to the rotor and an increase in spiking on the motor lines.

If all the above are good look at the dc supply for open diode in a bridge,(There may be two bridges one for field and one for armature supplying different voltages) high impedance connections ect. These can increase spike levels and in the case of open diodes lowering the voltage availably drive the motor current up to compensate. hope this helps Terry