idle problems

I have an 84 200sx w/ the 2.0 and every now and then while i am chillin at a red light or something with the clutch in or in neutral, the revs will just drop, it will idle very low and sound bad, sometimes it goes below 500, which is the smallest reading and if i do not hold the pedal down to keep the revs up it will die. I have talked to some people and all I get from them is different sketchy answers, i was wondering if anyone has had the same problems and or what to do about it. I also was wondering if anyone had some good ideas for ways to make my fuel efficiancy better, of increasing the power and acceleration. Also does anybody know where to get parts for my kind of car? like body kits and stuff?

I pretty much have the same problem.. especially when the car is cold. I can rev it up, and then it goes down to about 4-600rpm. After it warms up the same thing happens, but then it will first go down to ~500 and then rise up to 900-1000rpm.
One strange thing though, is that the idle rpm seems to be rising with temperature. So last time i drove hard and the temperature went up to maybe 3/4, the idle was at ~1300. And then went down to about 1100 with lights on (?) and 1000 with lights off at normal temperature.
Haven't got a clue what to do about it..

I was having the exact same problem a couple of days ago. I had just finished doing a tune-up on the car and when I went to crank her over, she turned on like usual, but after about a minute, the rpm's dropped below 500 and the car died unless I gave it gas. I figured my fuel pump was going out, so I tested it, nope it was fine. Then I though maybe the distributor had whacked out on me, tested it fine. Then I started thinking lack of air, so went through the entire intake system, except the tubing before the air box, to see if anything was clogged. From the filter all the way to the intake manifold, there wasn't anything. After a full day of just troubleshooting, I called it quits and decided to start again in the morning. By this time I had no idea of what could be wrong. In the morning I started problem solving again, I happen to drop a screwdriver and when I reached for it, I happend to look at the opening of my intake system and there was a paper towel stuck in the first bend of the piping. I was like " that doesn't belong there ", took the paper towel out and turned the car on. That was the problem one piece of towel. Needless to say I felt a bit humiliated.

Anyways, some things to look for:

1. Make sure NOTHING is blocking the air supply
2. Make sure the fuel filter is not clogged and that it is receiving fuel
3. If fuel filter is not receiving fuel, test fuel pump ( fuel pump should have a voltage output of 11.5 - 12 volts, another way to test is to have someone turn the key to the ON position, while you listen for the pump to engage)
4. Make sure plugs are gapped correctly, btw the s12 likes NGK plugs ...
5. The last thing to do is to look at the distributor

It runs OK at some throttle, and I've tried removing the air filter... idles still... lite crap :/ But i found this badly fitted hose, probably a homemade repair of something, attached to the idle control thingy at the front of the intake.... gonna check everything for leaks today...