Vehicle jerks and engine light comes on. Vehicle loses its speed to 20mph?

While driving the vehicle gives a jerk (esp. when gear transmission is made) and engine indicator light comes on.The vehicle at same loses its speed to 20mph and the sound of engine is total different. The mechanic at Isuzu dealer has replaced the fuel rail pressure sensor and fuel rail regulator which cost me huge money. But even with these replaced parts, the vehicle was for few days and same problem started to happen again.