Hey Coffinmaker. Glad you chimed in on this one; I was hoping you'd have some ideas. Here's the latest idea I had about it:
Curtis264, do me a favor and cycle the rifle slowly while empty. Look and see if the follower tab moves or wiggles, indicating contact between the follower and the elevator. If it doesn't move at all, that may have some bearing on your problem.
As you probably know, cartridge overall length is critical to these rifles. Too long, and they won't even fit in the elevator window in the frame. And they can't be too short either, but there is more tolerance for short today than in original rifles.
When you have a stack of cartridges in the magazine, the round in front of the one in the elevator will have its rim protruding into the elevator window. That bevel on the front bottom of the elevator ramps that cartridge head back into the magazine as the elevator rises. Original rifles only had a shallow elevator bevel, expecting cartridge length to be held to tighter tolerances. Today's cowboy ammunition is often a bit shorter than the originals, so the rims protrude further into the window. Uberti has compensated by making that elevator bevel deeper. This compensation has a limit, as too deep a bevel will cut into the cavity for the lifter arm.
Anyway, as the cartridge rims are being ramped back into the magazine, it is possible for the rim to catch on the top of the magazine opening. So I always bevel and polish that area as well. A lot of folks don't do this and experience no rim catching, so I don't really know how important this is; I just do it to mine.
My theory on your rifle suggests that your cartridge follower may not be protruding far enough, which would only be relevant on the last round. That last round is free to rattle around in that elevator, because there's no case head on top of it pushing it back; there is just the follower to limit its forward movement. And if your follower doesn't touch the elevator, and that last round has rattled its way to the very front, there might be enough space for the nose of the bullet to contact the top of the magazine entrance when the elevator tries to rise. And if that area isn't beveled well, the occasional bullet nose may jam up there until you push down on the rim, likely moving the whole cartridge backwards a bit and pulling the nose away from that area in the process.
Just a theory. Wish I could examine the rifle in-person. If the follower isn't contacting the elevator at all, you can test my theory a bit by levering with the muzzle pointed skyward. If you NEVER jam in that position but sometimes jam when horizontal, I think a short follower along with a poorly-beveled magazine opening may be the reason.