For the past 6-7 weeks, I've had a sick '04 D2 HSE with 190K miles, that exhibited the following symptoms at cold start:

It would start
then it would rev momentarily to 2-3,000rpm
then it would drop to 200-300 rpm while "belching" (backfiring) within the exhaust manifold (or at least, that's what it sounded like)
It would stay at a few hundred rpms for several more seconds before it would stall without any accelerator intervention (feathering, pumping)
Once warmed up, it would start normally until it cooled down again

Finally, during those cold startups, the SAI pump didn't run even though the conditions for it running were met (air temp >7C, coolant temp <55C).

So after posting several threads on this forum, almost 2 months of troubleshooting, changing the following:

CPS
Fuel Pump
Coil Packs
Ignition Wires
Spark Plugs
etc.

and testing a bunch of circuits and relays for signal, continuity, etc., I finally circled back to the first or second piece of advice I had gotten on this forum to my thread to check for vacuum leaks (I did a visual/tactile inspection of all the lines and saw nothing obvious so I moved on to all the aforementioned other expensive stuff) and decided to pull out my homemade paint can, mineral oil smoke machine (if anyone wants to know how to build one, there are youtube videos on it) and smoke tested it.

Viola, that elbow part of that fuel line style connector on the hose from the EVAP purge valve to the intake manifold had failed, and I had a large, non-obvious vacuum leak.

After fixing it, the beast runs great again.

Moral of my story: don't dismiss the obvious -- and listen to the smart guys on this forum.

Or in the words of Spock:

When you eliminate the impossible, whatever remains, however improbable, must be the truth.

 

Thanks for passing on your knowledge.