That's exactly what I was going to do. Looks lean, I'd check the O2 sensor with a digital storage oscilloscope. Kalifornia states that the response time lean to rich should be under 100 milliseconds. In reality it should be in the 30-40 millisecond range. Any slower than that and it can't truely control fuel properly for best emissions.

BTW, running lean can burn out a catalyst almost as fast as running rich. More unburnt fuel (lean misfire) gets sent to the cat with extra O2 to make sure it burns well. To top it off, running lean means running hotter. The excess heat contributes to catalyst substrate degradation.

My suggestion is for you to take a hard look at that O2 sensor.