The problem with your generator is in the AVR (Automatic Voltage Regulator). This regulator is an electronic device that mantains the output voltage at the designed level (120V or 240V ). A surge protector is not a solution.
The AVR is located near the end bell of the generator, near the brushes . To this electronic card are attached wire from the stator and to the brushes. Normally the electronic components are covered with epoxi resin, so they are very difficult to repair and very easy to replace.