Nissan Serena cutting speed

Something strange is happen with my car, when it is warm it cuts the speed. From this moment it's very hard to drive, because the doesn't move fast i cant stay at 20km/h at 3000 rpm. While this is going to happen, I feel the accelerator pedal as if the wage is slightly harder, if I take the foot off the accelerator, the car starting loses its power and acceleration as if something prevented it to speed. somebody told me to change the speed sensor to solve the problem