I had that problem and it was the ESC firmware. The problem is that the firmware does not do a good job of quickly starting the motor with a lot of power. You must start it gently.
Sitting on a little slope, the lower motors would not start because they wanted to start with more power because of Attitude mode.
My temporary solution was to manually tilt it so that the lower motors were not lower any more. You can tell which motor has the problem. Just zero the throttle, make the stuttering motors a little higher and try gently to start the motors again. After a few tries you will get all motors running at the same time. For me, the motors ran fine after that.
The permanent solution was to flash the latest firmware (BLHeli or SimonK).