You can bypass the IACV with a jumper. I think this is typically done when setting the base idle mixture as well as setting idle speed.
This thread asks what a jumper is but tells you which pins to jumper
http://forums.pelicanparts.com/porsche-911-technical-forum/188748-what-jumper.html
When you're testing the idle switch you're rotating the throttle by hand and testing it with a meter to make sure that it's opening/closing right?
Don't open up your AFM. The car was running fine and passed a smog test not long ago so I doubt the tracking went to hell that quickly. Plus there's a way to use an ohm meter to test the pins that will tell you what kind of shape the AFM is in.
Normally an idle issue is related to unmetered air but you've replaced the intake gaskets (assuming you also looked at the intake spacers) and you've verified the vacuum side is tight.
You mentioned a stumble at speed I think. That would take the IACV out of the equation. Might be worth having a look at the boards in the DME. The IR thermometer is a good idea for a simple check and they're just handy to have around in general.
Here's a thread that talks about the Jumper and setting the idle speed. Beware of Zao's avatar. It's mesmerizing
http://forums.pelicanparts.com/porsche-911-technical-forum/369102-idle-adjustment-hex-screw-without-jumper.html
Here's a thread by Sal on solder joints. Loads of threads on DME's developing cracks or bad solder joints but this one specifically mentions 3 for the transistor that drives the ignition coil.
http://forums.pelicanparts.com/porsche-911-technical-forum/652375-dme-bad-solder-joint-repair.html