I don't know if its possible but maybe your left and right upper oxygen sensors are mixed up? I would think if that was the case the computer might see that bank 1 is running a bit lean, but because its the wrong sensor it would actually be reading bank 2s output and then pump more fuel in causing it to run rich in the wrong bank and creating a cycle of rich bank 1, lean bank 2.

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1993 Ford Ranger XL, M5OD-R1 5-speed, 3.0 Vulcan V6, 3.45 open diff, very beat-up, bought for $375.