What wrong with the engine when the idle go up and down 240sx?

the AIV valve is dirty. This is a very common problem, your best bet is to remove the entire AIV box (located behind the airbox on the drivers side) via the 3 screws holding it in place, then remove and plug the 2 hoses that were leading to it. This is completely safe, and all it is doing is keeping exhaust fumes from entering your intake manifold. If this doesnt work, then try installing a new fuel filter, preferably one for a 1990 300ZX, it is a direct fit and will give you a better throttle response