I have had the same issues with surplus ammo in mine. I installed a extra power hammer spring and the issue has gone away for the most part. I end up testing each lot of ammo to see if it is going to work in the AR.
MGI makes a enhanced 7.62x39 bolt that uses a heavier firing pin, but I haven't gotten around to buying one yet.
I haven't had any issues feeding with magazines 10 rds or smaller, but the 30 rd ones are hit and miss for reliability. They also look strange with the bend at the bottom to accommodate the 7.62x39 rds.
What I am planning on trying later this year are 6.5 Grendel magazines and see if they work any better.
John K