98 240sx Rough Idle & Diminishing Gas Millage

Hi all. Hope someone can help.
About 3 weeks ago my 240, with manual tran, started idling rough. It seems to happen once the car warms up. It will idle right around 650 - 700rpm as normal. Then it will start bouncing between 1000 and 500 rpms and gradually getting lower until it sometimes stalls. This is inconsistent though. It'll idle rough at one stop light, then idle fine at the next one. I can also occasionally feel it stutter while I'm accelerating or holding a speed. My fuel millage seems to be getting worse, as well. I don't have my mpg logged, but before I was getting about 400 miles to a full tank and my last fill up I don't think I even got 250 miles out of.

Here's what I've done so far:
Changed oil & filter
Changed plugs
New Distributor cap and rotor
Changed Air filter

Mind you, I'm not terribly experienced at auto repair, so explain to me like I'm a 5th grader.

if you havent figured it out yet, i would check for a stuck fuel injector or a intake manifold leak.

I would check that the aic is not stuck in the open position. The aic is basically an egr valve(exhaust recirculation valve) but since its a Japanese car it has a different name. Any how, if you dont know what an egr valve looks like, have tour dad look in the engine bay and ask him to point it out to you. It is located down lower left of the engine head on the passenger side. I had a 240 with the same problem ...it would run fine till it warms up then start to run like crap unless the rpm's were up over 2 and a half thousand, then it would smooth out