ive got a 90 240sx with a stock ka24e, 113k miles. a new problem came up yesterday that makes the car undrivable. when the engine is cold it will drive fine for 5-10 minutes maybe but when it warms up the rpm will start to surge up and down a 1000 or so making the car shake badly. it will start higher in the band and move down till the car will barely run till its cooled off.

new things:
plugs
wires
rotor
cap
headgasket
timing chain, tensioner, guides
tried different injectors
o2 sensor

im waiting on a new fuel pump, thats the only thing i can think is that its dying on me. but im not familiar with nissan engines so if anyone knows something else that would cause this please let me know. i tried the nico forums but im not getting much help. someone guessed timing chain or timing was off but i dont think that can be it since it ran good for 2 days then this problem popped up.