well, my J30 idles at 400 rpm at times and it used to shudder (rumble) when it sat at 400 until i replaced the air filter box with a 4" cold air intake. now my car just sits at 400 and i cant even tell if the engine is running at a redlight but the rpms show that it is. on the fact about your alternator, um
DUH
, your lights are going to dim once your car is off. they are using your battery's energy which is not being recharged by the alternator. its a little bit of common sense if you think about it
. and quit worrying about so many things. your car is doing fine. just make some easy changes. i only really know the basics about a car but ive gotten my J30 to perform alot better with the simple low cost enhancements. oh yah and btw, you car should drive fine if you are a normal driver. i push my J30 to the limit almost everyday and she still runs like a dream. had a check up done on it recently and she checks out with a perfect report. 12 yrs old and it still runs like it was new. just worry about things like your tires and make sure to keep your oil changed every 3000 miles (or that is what i do anyways). do this and your car will love you and keep it shape for a good many more years. <-(Bad Grammar)