I've driven the car the last 2 weeks on the stock elbow and skinny O2, ran perfect. Drove it to the garage, installed a 3" exhaust, 3" test pipe, already had a 3" downpipe, and a divided turbo elbow. The fat O2 sensor in the elbow was some random thing; the guy I bought it from said he thought it was from an 86 300ZX and would probably make my car run like shit, but I had to drive it home with that sensor because the skinny wouldn't fit
I mean, it has to be the O2 sensor that's causing it; more exhaust flow wouldn't make the car do what it was doing.
It actually ran fine for a while, up until I got to a stretch of road and punched it. It boosted up, got up to ~60MPH, and then fell flat on its face; sputtered and wouldn't go over 3000RPM the rest of the way home.