So, since I posted last, I've tested the gun with a variety of springs.
The problem still occurs with all original springs. Well, all original springs except one - the original extractor spring was ridiculously squashed and provided hardly any tension, and the Wilson and Wolff springs were way too short, so I replaced it with a stiffer, longer Wolff spring (magazine disconnect I think) that provides an amount of tension that is usually recommended (difficult to move with a fingernail). There's also a new sear in the gun, since the old one allowed the hammer to drop while on safe. The jam occurs with multiple magazines, and multiple ammunition brands.
The jam is as described as before, either jamming before the shell is dragged far enough back to eject, or the shell is run forward again before it can be kicked out. The jam is MASSIVELY difficult to clear, and usually requires a hammer, or slamming the muzzle of the gun on a hard object. Here's a picture:
So now that I've determined that the gun is not oversprung, (and I'm doubting it was oversprung to begin with, as I plugged Wolff springs into 2 other guns with no ill effects) I'm at a complete loss. Any ideas?