2001 Buick Regal Mysterious cylinder 2 misfire code

Check engine soon light flashes, indicating emission problem. Code reader reports "cylinder 2 misfire" but nothing else.

Replaced catalytic converter (old one badly clogged), all 6 plugs (with new Bosch Platinum +4s), all 6 plug wires (new Bosch premium), coil for cylinder 2, fuel injector for cylinder 2 and now the oxygen sensor before the converter. Oh, and also the air filter and fuel filter just for grins.

The only apparent things still left to replace are the mass air flow sensor and the o2 sensor after the converter. My mechanic believes it's the air sensor.

As far as symptoms go, the car was running fine before but now sporadically starts sputtering and missing a bit at idle (usually when it's first started or I'm stopped at an intersection). It actually seems like it might be a bit WORSE since we changed the upstream o2 sensor this afternoon.

Anybody seen this before or have any ideas?

Thanks!