Mechanic traced the problems with my misfiring over to the AFM. He said it was reading way too much airflow and hence compensating by getting the injectors to open longer. This also apparently also caused the mild hunting and occasional dying of the car from idle. The diagnostics that was showing does seem to confirm that.
He then swopped AFMs and now the car does seem to run smoothly. Unfortunately, it does seem to feel much more sluggish in throttle response too.
Now, some time ago, I went to another tuner and complained about low and mid-end sluggishness. He did something to my car which noticeably helped but he kept mum on what he did ("tuning secrets..."). Do you think its possible that he meddled with the AFM? Mine's a C20XE, by the way.