Unique idle problem

[Charles Darwin]

M42 has a little cammed V8 rumble at idle. I know theres a million threads that pertain to idle issues so I'll try and make it simple.

Symptoms:
V8 cammed idle (minus the V8 power heh)
Gets better when warmer.
Have to hold the throttle on initial cold start when it's cold outside.
When engine is warm, sometimes there is a long start.
On these long starts, often I can crank the engine over forever and the car won't start until I give it gas
Stomp test does not work.
Car actually idled pretty perfectly with the O2 unplugged, this is true when I have a new O2
Car idles the same with FPR unplugged.

Things replaced:
Mess under the intake has been done.
New ICV
New TPS
New Coolant Temp Sensor
New Cam Shaft Position Sensor
New O2
Tried different ECU's
Tried different AFM's
New Injectors

What I suspect:
FPR.
Check Valve needed at the fuel pump.
Harness issue
Crank Shaft Position Sensor


Please help. Losing my sanity. It's been like this for 70,000 miles 

[Charles Darwin]

Also, have done a smoke test, no leaks (as I suspected).

[Tgoode318]

Was the o2 you replaced a OEM one or aftermarket?

[Charles Darwin]

Was the o2 you replaced a OEM one or aftermarket?

It was OEM.

Update: After many tedious attempts, I got the stomp test to work. I've owned many E30's that were all easy to stomp test, this one took quite a lot of tries to get to work.

Stomp test code: 1215 - AFM. I have tried different AFM's in the past and they have not done anything. Will check the wiring behind the AFM boot but I am sure it is good. AFM carbon tracks were also in good condition last time I checked.

My suspicion now is a clogged cat converter. This is the original cat converter on a car with 300,000 chassis miles.

Has anyone actually had an issue with a clogged cat directly effecting the idle of the car?

If it helps, my car produces an awful amount of backpressure, you can really feel the exhaust flow hitting your legs, but I'm not sure if that's normal on these M42's

The reason why I believe it wouldn't be a clogged cat is because when I disconnect my o2, after a while, the ECU will adjust and it will idle relatively perfectly. My o2 is brand new btw, and this would happen with an old or new o2.