My guess is the the mag spring, that Z-shaped spring under the follower has been bent or otherwise messed with.

I took a 77 to the NH plant because a "smith" lost safety parts when doing some work for me. While at the plant I mentioned that after firing a shot or two,the unfired PSP bullets left in the mag were slamming into the front mag wall and getting damaged. I was told it was the mag spring.

That could explain why the follower is not bearing properly against a cartridge or two, and they seem loose. I can't say for sure why a cartridge would pop out, but it could be a related problem. Trying to recall what the mag lips look like, but can't. Also can't see how those would get damaged, if they exist at all, they do not come out of the gun when you unload via the DBM.