Gun locking on full

[FraserG]

Hello everyone!
Recently I've had the problem that when firing on full, after a few shots the gun will start shooting semi, even when it is on full with the selector switch.
The gun is a G&G CM16 Carbine, with no internal upgrades (I haven't even touched the gearbox!) and I was hoping someone could give me some pointers on troubleshooting this.
Thanks!