Sounds like there could be 2 issues.
1 - since your quad is not staying perfectly level you should try run the accelerometer calibration again, and if one motor stops completely when you yaw, sounds like you might have to do a ESC calibration as well. If you place your quad perfectly level (you can verify using a spirit level) and it should show at perfectly level in librepilot GUI when the accelerometer is correctly calibrated. As for checking the ESC, when you power on, all motors should start spinning at the same time, and at the same speed. For more details on setting it up check out the video that @Sam made:
2 - the other issue could be a faulty connection between your motors and ESC causing it to cutout randomly during flight. The best way to check is to remove the props and apply some throttle to your motor, then wiggle the ESC-motor wires to see if it causes the motor to cut out at any point.
Hope that helps