May also be good to know what ammo was being used. If that's the load you want it to run make sure the rifle is properly lubricated, use quality mags in good shape (PMAG, quality GI, Lancer) and you may need to drop down a buffer weight (i.e. H2 to H1). Also if you've made any changes consider going back to factory configuration. Also check to make sure things like the gas block is aligned and hasn't moved, the gas tube roll pin hasn't walked out, etc.
Fire the gun with different magazines to try an replicate the issue. How many rounds have you run through the rifle? Have you added any thing to like a bad lever? I have one issued to me so I know how they are ambi but I have to ask?