Had a similar problem running extremely rich "all of a sudden". Turned out the wire shorted at the junction to the O2 sensor. I would check all the wiring related to the O2 and any other sensors.
Come to think of it, I had the exact same problem at one time. Unplugging the O2 harness works too. It'll throw a check engine light, but it will take the O2s out of the equation.