replacing the sensor as part of the solution, so it doesn't really matter whether the sensor is bad or clogged with crud because "cleaning" the sensor isn't really an effective option if it is clogged. It needs to be repaced.
However, running rich, stumbling could also be a symptom of an vacuum leak and the o2 sensor is just reporting the rich condition of the exhaust gas.
If you want to lean out your car you could disconnect an injector or two.