Turbo overboost means the car's sensors see too much turbo boost and send the car into limp mode to prevent any damage.
It could be vacuum lines, turbo actuator, turbo vanes, faulty sensors or wiring, many possible causes. Find out exactly what the code is and datalog the turbo boost for some more suggestions.
Since you changed the boost pressure regulator it's probably not that. It's probably not the throttle pedal because there are check sensors there and if it were bad you'd be getting error codes and 1200 rpm max.