I'm scanning a local's 2001 SSEi Bonneville and its got an ugly cough under light or full throttle.
Many variables take a nasty drop on the chart such as timing, injector duty, o2's, and sometimes the rpms.
The owner already replaced the front o2, coil packs, fuel regulator, and fuel filter.
Please take a look at the attached scan and let me know if this makes any sense.
Here is a sneak peak: