Recently, my car has started running really rough when it first starts in the morning. The throttle is very unresponsive, and under light load below about 1400 RPM (i.e. going up a hill, lightly accelerating, etc.) it stumbles pretty badly. It's weird, at around 1400 RPM it instantly picks up and feels fine. For testing purposes, I've pulled timing way back below a certain RPM, and that felt very similar to this - bog, bog, bog, then instant smooth acceleration. Also, it feels like the engine kind of jolts immediately after I add throttle (possible related to the acceleration enrichment function?). After the engine runs a few minutes (haven't been able to watch the datalog, but it seems like it's when the engine goes to closed loop), it seems to go away entirely.

I initially thought it was running lean, so I gradually increased startup enrichment to about 15 percentage points higher than stock, but that didn't seem to help or hurt. It doesn't smell abnormally rich, so I'm hesitant to blindly decrease enrichment.

I've made a few changes recently that could have caused this. I put long-tubes on it just over a month ago, but I don't recall seeing this behavior immediately after. A few days after adding the headers, I drove out from CA to school in VA, and I'm wondering if the humidity or cooler morning temps could have something to do with it. Lastly, a few weeks ago I discovered my PCV system was set up backward (the check valve was on the wrong side, causing the air to flow the wrong way and a strong vacuum in the crankcase), so I switched it to the proper orientation.

Looking back at my BIN dates, it looks like the stumbling popped up shortly after I fixed my PCV system.

Do you guys have any idea where I could start with diagnosis?