To start, I've posted a pretty thorough description of the problems over by turbobricks, but failed to entice any responses...and then before heading to bed remembered that I completely forgot to post a question off this way, as well :D Link to the specified thread is as follows:

http://forums.turbobricks.com/showthread.php?t=53906

Anyway, the problem is simple: The idle on my NA 1987 740 wagon sucks. It sucks a lot. By that, I mean when cold, it tends towards stalling and running rather rough. From what I can tell recently, the car is running on only two or three cylinders at low RPM. The exhaust note also matches the feeling of there being only two cylinders firing. The IAC valve is holding to either the open or closed state. (Any way to test this? A test point is mentioned all over the place, but no specifics as to where this lies or how to use it are stated)

Parts replaced thus far (in no particular order):
1) IAC Valve (New valve causes the engine to surge to near 2000 RPM and hold that, albiet smoothly, so old one was swapped back in)
2) Fuel Injectors (Swapped in from a newer model, yellowtops)
3) Throttle Body (the old one had a potentially faulty TPS, but could not take the car off the road long enough to adequately test (and repair it as needed), so I just bought a visually superior part from a yard. Installed, and idle is now 100RPM higher and notably more stable. Was the TPS possibly contributing to the problem?)
4) Spark plugs (though many months ago. at least two months before the problems began in the first place. A potential cause?)

This leaves me with what?
1) Fouled plugs
2) worn out cap/rotor/wires (Note: Unplugging cylinder 1 causes no change in idle whatsoever, meaning that it's likely not getting a spark. Will check plug's condition this Saturday)
3) Ignition amplifier? (unlikely, but it's possible)
4) Power stage? (also seems unlikely)
5) ECU? (Extremely unlike, but it was once unplugged during the summer to make changing the blower motor possible)
6) Unmetered air sneaking in? (Tube may be cracked...sealed the suspect area with duct tape and glue which seems to seal it fine. any better ideas until I can find one at a yard?)

So there you go. Hopefully someone can come up with some answers, even if they're completely unrelated ;)

Edit: I'd also like to point out a recent find:
My CB Radio picks up a *TON* of noise from the engine. Far more than what I've found in other cars, and with the antenna mounted off by the tailgate on the roof.

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1980 240DL, 1984 240DL, 1988 740GLE, 1993 850 GLT