This one's had my head scratching. I've got a turbo setup, with an AEM ecm, and 440 injectors. If the injector duty cycle falls to less than 15% while cruising, the car runs like total piss. It sputters and starts stumbling. This usually happens around 2500 in all gears with 3%-10% throttle. If I give it some gas, get the revs up to around 4000, and then let off the throttle, it'll also sputter and stumble although the injector duty cycle is up around 20%-25%. I pulled the plugs and noticed it was running lean, so I bumped the fuel pressure up some. It didn't help the stumbling any unfortunately. I borescoped the cylinders and didn't notice any major detonation. If I'm cruising out of those rpm ranges the car runs fine. My next thought was putting an analog meter on my throttle rheostat and checking for any unusual deflection. Beyond that though, I'm stumped. Fast Idle Control Valve maybe? I did notice it happens more after about 5-10 minutes into driving.