Problem with DC motor

I built in my project a dc motor. In my project I have to controlled on the velocity of motor-cycles.Part of the Circuit is programmed software with components which are proportional controller and other part from hardware components.When I check my velocity of the motor-cycle,the exist value of the DCMOTOR is not stable and it has an error of (+) and (-) of 9.5% from the wanted value.More thing:the motor dosen't turns in stable velocity and its differential values of turns. How can I make the turns will be stable?