Car wont stay running....
 

I have a 1989 240sx with a rebuilt KA24E, it has about 15k miles on it, stock except for catback exhaust and Z32 fuel filter. A few days ago the car started to idle real rough after start up (400-500rpms) but after it warmed up a little it would idle where it usually does 750-800. So I changed the O2 sensor and replaced part of the wire because it looked cut I also replaced the water temp sensor. Next day I went out to start it and it start it and it did the same thing, and when the coolent gauge went to normal operating temp it ran fine.
Today I leave to do some stuff and at every light the car idles at 300-500 RPMs making the car shake like the engine wants to die. So I have to keep the rpms up so it doesnt die.

Now a couple days ago I check the spark plugs and they were white. Now at the time I didnt know what that meant, but now I do. Anyway after replacing the O2 and water temp they are a brownish color now. So I am not sure what that means. So if anyone has any ideas what could be wrong let me know ASAP. I also went through and check vaccum lines tonight and they are all connected and as far as I could tell all the electrical connections are made.


Also one more thing I did find something not connected, looked like a sensor by the oil filter, i know this may be a noob question but what is it? It had one wire connector and i saw the wire a few inches from it.

fresh gaskets on rebuild? cuz you got funny colors on the sparks... i can't help but think leak somewhere....