You have any pics of it? From what I understand, the fuel injectors throw fuel in right before the intake valves, right inside the intake manifold. Over time the intake manifold will get gunky and varnishy from fuel and stuff. Try running some seafoam through it, and really clean it carefully and well. It's very common, and happens to all cars. Probably nothing bad :)
I don't think a bad piston ring would cause your intake to gum up, because if your intake valves are still good, they wouldn't let anything through on the compression or exhaust stroke. If your problem persists, I'd look at the valves, make sure they're seating nicely like they should be. Has your car ever been overheated, or anything like that?
Your stalling problem could be tied to a dirty IAC (idle air control valve). If your intake is dirty, chances are that thing is just plain grody too. There are various how-to's on this forum, with pictures and great advice on how to clean that bad boy.
Check for vacuum leaks. I'm checking for some on mine, too lol.
Check/clean your Mass Airflow Sensor (MAF) and make sure it's in the right way.
Check your air filter, and all your PCV's and PCV hoses.
Any check engine lights?