Jiganto

The car has about 54k miles on it.

About a month ago, the SES light came on. I took it to autozone, and it was running lean. I replaced the PCV valve, cleaned the MAF and checked the voltages on it, and it was running fine. It ended up being the first oxygen sensor, which was replaced. Because the car had weird idling problems before, especially in the summer, where it would stutter when I first got it moving from a cold start (hasn't done it since the O2 sensor has been replaced), I decided to replace the spark plugs too. I replaced them with generic NKG plugs, all gapped properly and all. Since then, the engine oil has also been replaced one and the radiator has been flushed.

For a month the car ran ok, however it had this other odd idling problem now. Where after it was warmed up, and I was standing still with the engine idling the engine would periodically dip about 250rpm every minute or two, which you could see on the tach and could also hear. Then yesterday, while idling at a parking lot (still periodically dipping) the SES light came back on. I took it to autozone today, and this time the engine is running rich.

I suspected that it was faulty NKG sparkplugs, so I bought a new set of BOSCH ones and replaced them. Well the light is still on, and the engine still does the dipping thing :-/

Any advice on what could be causing this?