Here are my suggestions:
1. Could be ammo related - have you tried using a completely different kind of ammo? Are you using reloads? What ammo are you using? Look at the primers - are you getting light primer strikes?
2. If you are certain it is not ammo related, then my next guess would one of three things: worn firing pin, worn auto sear or worn full-auto bolt carrier (where it trips the auto sear). Dissassemble the bolt and inspect the firing pin (also make sure there is no material stuck in the firing pin hole). It should also be pretty easy for you to check the timing of the FA mechanism with the top cover off and no magazine. Just tie back the trigger and cycle the action - you should be able to observe whether the auto sear is working correctly.
3. Also check where the auto-sear engages with the notch on the hammer - if this is worn, the hammer could be riding the bolt carrier, which would cause symptoms like what you describe.