s54 swap experts needed (running issues)

[Kondata]

I'm probably not smart enough to relay the problem correctly but here's a quick summary:

2000 m52tu z3 chassis with a s54 swapped in. Bought the car this way. car exhibits signs of what can be considered misfire with rough stumbling all through the RPM. cold start will typically result in car starting idling for a second, then dying. Do this a few times until the car catches idle and warms up, then it will idle (albeit rough). Sometimes it idles smoothly and revs clean, but ultimately stumbles. No misfire codes. Low power most of the time with random bursts of healthy revving.

Only codes present:
P0121
P1637

When I got the car, the MAF was plugged in backwards. I installed it in the correct orientation only to find the car runs significantly worse with MAF in the correct direction. I replaced the MAF with known good sensor. No change. I replaced front TPS with known good sensor, no change. I reset adaptations only to find that the throttle pedal now did nothing. If MAF is unplugged, car will only throw a code for the IAT sensor and not the MAF itself. We ran a smoke test to insure no vacuum leaks, non were present. MAf readings register on Snapon scanner.

Engine sounds mechanically healthy without any knocking or even loud valve noise.

I realize this is playing phone mechanic, but any suggestions from any of you S54 veterans would be appreciated. Car will be going to a local pro here shortly, but I figure if any low hanging trouble shooting options come up, why not try.

Thank You

[Kondata]

in case this helps someone in the future, issue turns out to be the throttle actuator and faulty ECU. We replaced the throttle actuator and car ran great without maf (where before it would throw various tps codes and would go into limp mode). Unfortunately, onc the MAF was plugged back in, it would exhibit the same symptoms (rough idle, surge, die constantly). We swapped to a known good ecu and car ran like a champ. Last unfortunate note, engine now developed what I think is a cylinder head knock (perhaps a worn or bad rocker). Looks like valve cover is coming off.

[PITT M3 RR]

Do you still have that bad DME?