Below are some common problems/answers.

This is probably due to a bad radio-connection (using same channel as WiFi?).

Most of the time this is either due to the fact that there's dirt (like hairs) stuck around the axis of the motor between the motor house and the propeller or that the propeller is too close to the motor so there's friction between the two. If neither helps check that the propeller is attached in the right way.

If the linkspeed is set to 2Mbit switch it to 250Kbps. This will normally increase/stabilize the radio reception. Since there are other devices (link WiFi) that might run on the same 2.4GHz channel try switching to a different channel for the quadcopter.

There's spare parts available at Seeedstudio