Well, I got my new drive from MakerGear, and installed it last night. Once I had the old one off, I could see the problem. Because the drive gear was too far back, probably combined with twisting of the bearing due to the offset support, the filament had been pushed forward inside. It wore a large groove in the top section of the drive:
and a slightly smaller one on the lower end:
All of this was happening inside the drive, where it was impossible to see without taking everything apart. Basically, the filament was coming down, and bending around the front of the drive gear.
One possible way to avoid this in the stock drive design would be to use a smaller bearing, with a grooved sleeve like in this thread:
http://forum.makergear.com/viewtopic.php?f=10&t=3210&p=23537&hilit=filament+drive+bearing#p23535The stock bearing is 5mm bore, 5mm thick, and 16mm OD. McMaster Carr has a 14mm OD bearing (#7804K109). It should be possible to make a press on sleeve with a small groove to keep the filament from wandering sideways.