Tricky Misfires, part throttle only, cyl 1,2,3,4
 

2008 Civic EX 5-speed R18 with 138,000 miles

I've been trying for months to diagnose the following problem:

Misfires consistently in all four cylinders ONLY under medium/part throttle (ie, "normal driving").
Idles perfect. No misfires at idle.
No misfires at Wide Open Throttle.
Does NOT misfire if I drive like my grandma (extremely light throttle).

No check engine light. No codes (none whatsoever). I have a scanner, and I can see all four cylinders are misfiring under medium throttle only. It happens almost every time I use medium throttle. Plugs changed at 108,000 miles (NGK Iridium). I'm open to ideas. I will give results from scanner live data, if you tell me to check something. The challenge here is to not throw parts at it... until we know what it is (evidence-based). Anyone up for the challenge?