Stutter or stumble under slow acceleration?
does it happen every time? id start with fuel pressure first. my car did that when it had a bad fuel pump. if its fine then go to spark plugs and see what happens from there.. good luck

EDIT: you might wanna check your fuel filter too!
I also had a problem that was open/closed loop related. At full throttle the fueling is taken from a table, so it ran OK. In my case I think 7 out of 8 cylinders were running OK.
At part-throttle/cruise (closed loop) it ran badly. Long story short, for me it was a bad injector. I think what happened was the bad injector made 1 bank of the engine look like it's running lean. To compensate the computer richens the mix on the entire bank, and then you have 4 cylinders with a bad mixture.
I think an exhaust manifold/header leak can give a similar case. I replaced the gaskets there before the injectors but it was OK.
If you have access to a logging tool like efilive you can see this case. For me in cruise mode 1 bank had 2x as much injector duty cycle as the other bank when it ran like crap.
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