Thanks for the advice. I spent some time searching for a vacuum leak this weekend, but to no avail. First I tried the hose-in-the-ear method, but couldn't find anything. Of course you can only get to about 20% of the overall surface area where leaks might occur with this method. So then I tried the spraying-carb-cleaner method(engine cold and fire extinguisher at hand, just in case) but no luck there either.

It'd be nice to have access to a smoke machine. If anyone has any other tricks, I'd love to hear them.

Also, there is one other symptom that I had forgotten about that might be helpful in diagnosing the problem. When I first start it cold, at the first stop sign or light, it seems that it's clearly "hunting", i.e the idle is dropping and then slowly climbing back up over and over. Once it gets warm, it stops doing this. This behavior has continued over a long period of time, so it doesn't seem that the ECU is ever convinced that it's got the system running correctly.

thanks,
steve