no power at low rpm when engine is cold

Can't answer your question, but if you follow this procedure you will more than likely resolve your problems. Print it out, and spend an hour or two going over it.
L-Jetronic

lol, I've never had a car that didn't do this (Alfa might not, when I put a radiator in it and run it, but we'll see).

I don't know much about engines, but do you have an automatic choke, and if so, is it working?

Of course, the original low-tech solution: rev a little higher than usual until it gets warm. Maybe stay down a gear from what you usually do, as long as that doesn't rev it too high.

When you say "unplug it" re: the Aux Air valve, do you mean the electrical part? - Not really a good test of the device i'm afraid. GreyGTV6 is probably right. You really should familiarize yourself with the FI injection system on these cars. It's fairly simple actually. Your problem could very well be the Aux Air device. It's hard to find one that works equally well at cold starts (like mine) and hot starts (not like mine). If you pull it and toss it in the freezer for a bit, then pull it out, it should be open. Let it sit out in ambient and check it after 10 minutes and see if it closed (may not close all the way). You can adjust it by loosening the little nut on the diaphragm while keeping the valve open a bit with a micro-screwdriver. The other thing to check is the coolant temp sensor. To check it you'll need a multi-meter, I think the value is 4-Ohms. Pull the connector and test across the electrodes. Hope this helps.