One thing I learned when troubleshooting is to start with the obvious. It's also a good idea to know what you are looking at;
1. Look at the LPK and make sure that someone did not install a real M16 FCG in the weapon. Trust me on this. A sear is not required to make an AR run full auto.
2. If the hammer and trigger are good to go, then simply replace the springs, to include the disconnector spring. Test fire it again.
3. If it continues then you need to look deeper. If it was out of spec the issue would have manifested itself sooner.
4. I would then closely examine the trigger and hammer. It may seem tedious, but it serves a purpose. You learn how to troubleshoot and you don't spend alot of money unnecessarily.

