s14 RB20DET revving problems??

[3v1Ls14]

yes have searched on google and all sorts of Nissan forums

..and yes i know its a problematic RB

Here's how it is..

Starts ok and idles ok, has had slight miss and stopped firing one cylinder, new plugs and fixed the miss.

It will idle fine and rev ok, but wont rev past certain rpm almost like a fuel cut. try driving it and soon as its about to boost it chuffs black smoke and loses power, but doesnt die.

fuel pressure is good, 40psi at idle or just under. have tried known good maf, known good ecu. injectors all working well and have specified impedance. I have continuity tested every wire in the loom from ecu plug and checked for accidental earthing out in ALL wires.

tried unplugging tps, same result, o2 sensor, same result, afm, same result, idle valve, same result, temp sensor, same result.

i am getting ecu code 55 so no fault. new plugs, good afm, good ecu, new temp sensor, standard turbo, good fuel pressure.

no matter what i unplug the conditions of how it is running do not change.
even if i unplug afm and try rev doesnt die or anything. just revs up a bit then hits the cut EVERY SINGLE TIME. getting very frustrated and running out of ideas. dirty fuel? air leak? any ideas or suggestions would help. cheers guys