Car stalling ... ECU 21 code

[Drift]

Hello, my 92 240 is pissing me off. The cars starts and idles like a champ. 50 percent of the time it runs beautifully, now for the other 50 itsat 500 ****. My car is so unpredictable. Bad mode: RPM cutoff is set exactly 0 rpms, every time this occurs. Imagine if you disconnected your AFM harness and how it stops at exactly 2400 rpms, it does that at exactly 5000 rpms. Really bad mode: Cannot rev past 2100, if attempted rpm meter flails erraticly, but no acceleration. Car will drive fine under 2100 until its revved, it will still idle normally. It almost always does this when it is hot outside after driving for about 10 mins, never usually during the evenings - it does however occasionally. And it switches back and forth betweed bad mode and really bad mode. Lately more really bad mode then other. The check engine light is not on // but when I did check it its throwing ECU code 21 only. Per FSM the problem is the power transistor module or the harness, I replaced the coil, module, and the harness // same problem. I've replaced the AFM, FPR, fuel pump ... any ideas? I'm starting to think ECU, but FSM says its rarely ever the case ... but its seeming to be my case ... any suggestions? I'm really open because I've just been replacing fuel and ignition related things like crazy with no results.

[NISTECH]

well the code throws this thoery off but i would say you may be loosing your ground to the air flow meter. but in most cases it would throw an airflow code. try splicing in an alternate ground to the air flow meter and see if it helps. if not try loosening and cleaning engine and body grounds.