Ok - do you ever get the motor to spin up fully and only get stuttering at certain times (i.e., you slowly ramp up the throttle)? If so, I would lean towards it being timing on the ESC. I had that same issue when I was rebuilding my Sea Duck; somehow the ESC settings got messed up when I was trying to recalibrate and I didn't realize I'd gone into the timing. Once I reset the timing, the stutter went away.
What ESC are you using? Emax BLHeli ESC?
What ESC are you using? Emax BLHeli ESC?