What did your instructor think it was? Did it ever run right before? If so, I would first try replacing the outer recoil spring or the complete set, inner and outer with guide.
Sometimes, it's not just one thing; which can make it hard to pinpoint. As others have mentioned, it can also be the mag springs. Also, mine would start to hiccup after 150 consecutive rounds with dirty ammo that would form a muddy layer on the feed ramp. I polished the feed ramp and throat and that issue stopped happening. I take a rag on my pinkie and wipe the feed ramp if I'm shooting a lot. I've got about 4000 rounds through mine now and it's just sweet as honey. Other than routine recoil spring replacement, zero issues and 100% reliable now. Hope you can solve it. Let us know.