97 Tracer idle stalling

I did a search 1st, and found the topic: Merc Tracer idle problem, which sounds close to mine but not quite.
My 100000 mile 1997 Mercury Tracer with automatic has started stalling on
idle. But mainly in Drive and Reverse. However, while the idle RPMs seem to go up a bit in Park and Neutral, it can still start to stutter and die if I'm not careful.
Temperature doesn't seem to matter, whether engine temp or outside temp. It'll always stutter and stall in D and R. However, I THINK the idle slows a bit in P and N after it warms up...but I haven't been able to tell for sure yet.
I saw something about stuck air control valves, but that seems dependant on temerature as well. But in any case, can anyone tell me where to locate it and how to determine if it's working OK? (I can't really afford to get a Chiltons or other repair manual right now.)
What other possible causes could this be? The car otherwise has power and accelerates just fine.
Thanks for any suggestion!! Liam
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