Recently the car started idling unevenly. Higher RPM performance doesn't
seem to be affected much, gas mileage is normal. OBD code indicates 1-1-1
but I haven't been able to pull diagnostic codes yet (not entirely clear
how exactly to do that - probably a separate query post.)

The symptom is, right after cold start, idle is fine, or less bad.
Once the engine warms up the idle is bad, with in gear RPM around 700.
It sounds as if there are some misses, I can hear puff, puff, and the car
shakes lightly corresponding to this stumbling. I tried to rev the engine
up and found the uneven idle would disappear at around 1500RPM.

I will appreciate any advices on how to approach diagnosing and fixing
this problem.

Much TIA,
Lih-Yen

If I am not mistaken, 1-1-1 indicates no fault, which just means it isn't bad enough to show up on the OBD yet. I had similar problems and it ended up being a hole in one of the vacuum hoses. Definately check them all in addition to what skim35 had to suggest. From my experience, if it's sucking too much air, it'll do what you are describing.

Sam

First of all, I would remove all the spark plugs. Make sure the hole is clear if Gas, Oil, Water...

Chek the distributor cap, roter, wire afterward. If you have not changed it in last 2 years, replace with new. The are not too expensive.

Then clean out throttle body if you have not cleaned in last 3 years.
Check all intake air hoses, replace the ones cracked or leaking air.

Your car idle is controlled by Bosch Idel Control valve, you can take it out and clean it as well.