throttle increasing on its own

I have callibrated my ESCs, acc. meter, compas, & radio....but as soon as i arm the copter with props on it... it started increasing the throttle speed after advancing from idle speed of mot_arm_speed... and if not decreased manually the copter tends to flip either forward or in backward direction....
Anyone with the solution for this problem...... plzz help me out.
Waiting for your sincere advice.