Formerly reliable BCM 11.5" upper has sudden cycling issues

[1911-A1]

I just need to reason through this problem I've been having, because I can't seem to find the cause. It's worked extremely well for thousands of rounds, no breakages or issues to date.

My BCM 11.5" LW upper with A5 system developed a sudden reluctance to feed a new round after firing and extracting the old cartridge.

Actions taken:
-Used Gen2/3 pmags and USGI NHMTG mags. No change.
-Re-Lubed with SLIP 2000. No change.
-Used XM193 and M855 ammo. No change.
-Tried BCM BCG from another rifle. No change.
-Tried a third BCM BCG. No change.
-Tried different lower. No change.
-Tried 10.5" upper on original lower. Functioned fine.
-Detail stripped, degreased, cleaned and inspected upper. Worked fine for 10rds, then same issues began again.
-Lubed with white silicone grease. No change.
-Swapped A5 buffer and spring from other gun. No change.

I can't think what else to try. I could try a carbine buffer and spring, but since I already swapped out the A5 guts from another gun and had the same problems, I don't know what that will accomplish. I can't see any problems with components. I pulled the barrel off, removed the gas block and tube, no issues seen. Bore is shiny and undamaged. BCG seems to be functioning properly with no damage.