A real long shot here, though the air duct from the AMM to the throtle body can crack and cause this by allowing air into the engine that is not via the AMM. This very problem happened to me a few years ago, though it would still fun, just not well.
As a patch up some duct tape or race tape can keep you going until a replacement can be sourced.
Again, a long shot though worth checking. It could also contribute to the O2 sensor reading lean, because it is running lean.
