Hi folks,

A funny issue has been coming and going with my 900S ('88 nonturbo). 370k miles.
While running at highway speeds for some variable period, the engine will suddenly seem to lose power. From this point on, the car will not rev over ~2k RPM. This "rev wall" is very consistent each time this occurs, and the problem will persist after restarting the car. I have seen this issue disappear after a day - I've seen it stick around for weeks.

I replaced the fuel pump a few years ago.
I replaced the fuel filter right after this started happening.

In the months since I've moved the car around with this issue for street cleaning, it began to run horribly, often stalling during idle and running filthy. To address this, I've changed the oil, spark plugs (they were disgusting!), and lastly O2 sensor.

I've cleaned and verified IAC is functioning. AMM passed the test described in the Bentley's manual (the wire briefly glows red 4 seconds after shutting engine down), indicating it is likely to be working well.
Vacuum lines appear to be in good shape. Disconnecting ELCD makes no difference. I've even removed to the fuel rail to inspect fuel injectors and those look pretty clean from the outside.

Since replacing the O2 sensor, I am reading the following codes:

12111 Learning fault in lambda(air fuel mixture adj)
12112 Learning fault in lambda(air fuel mixture adj)
12224 Air/fuel mixture too rich

Before replacing O2, I read the same codes except 12223: "air/fuel mix too lean"

I am truly puzzled. I don't know whether to try replacing the AMM, and I certainly don't have a spare ECU on hand to swap out for curiosity. I would've thought "clogged catalytic" but the problem comes and goes, plus I replaced the cat a few years back.