I installed a high speed motor with stock spring onto my MP5k and had the same problem.
What I think happens is the motor spun with such speed that the piston is half cocked after every shot. Sometimes to the point that the gear timing would be off fooling the system into thinking the gun JUST fired a shot thus not firing. (Semi auto uses a gear and lever system so the gun will stop firing after one cycle of the piston.)
In full auto the motor spins no matter what so full auto would not be effected.
I am no doctor but can safely assume that this is the issue and nothing to do with lack of power. (if anything its too much power)
Only solution is mosfet