Fuel pressure regular checked swapped no fault, AFM tuned no change really.
Vacuum leaks meticulously checked(not smoke tested but well checked and sealed). All new leads resistance checked, and crank reference and speed tested ok. TPS good.
I think it has a lightened flywheel and hefty clutch, with a yuck rattle/chatter off throttle/coasting. Perhaps the lack of inertia could contribute to lumpy idle?
Or maybe the timing belt is a tooth off?