'90 240sx sputtering/hesitating, but want an opinion on something too

[theboy419]

I have a '90 240sx SOHC KA24E and lately it's been sputtering and hesitating. At first it was occasional so I wasn't too worried but slowly it happened more... What happens is, it'll come and go, but when it comes, it will idle weird(which made me sorta nix out O2 sensor, but i am gonna change it soon anyway), then if you hit the accelerator, the engine will sputter real bad and drop to about 400 rpms then sorta "catch" and suddenly rev high but sputtering all through until a lil over 3k. after that it usually smooths out, unless i'm on the highway then it may do it all the way up through 4-4.5k. It started only doing this more so at night, but I noticed it even happening today some during midday and afternoon. From what I've read it may be any of the following: spark plugs(a friend explained this to me, how at the low end the night air may be too cold and causing it to run too rich etc...), O2 sensor(which i bought but have yet to put in due to a missing 22mm wrench :-/), vacuum lines(which i recently replaced, but then took out the AIV which i've consided could be a part of this), MAF, coolant temp sensor, or fuel pump. If anything sticks out as painfully obvious it'd be greatly appreciated. One thing I have been leaning towards was fuel pump, since after 188K miles, i'm sure it's on its way out anyway, and i've replaced it on a previous 240 i owned anyway. The opinion i'm asking for in the subject is this: if i'm replacing the fuel pump, should i just get OEM replacement, or something like a Walboro? I know that's a bit much, and since i have stock injectors and pressure regulator, it could possibly cause problems i suppose, but I just wanted some opinions, since both fuel pumps are about the same price. My overall plan is to make this car a strong NA motor but that won't be anytime too soon thanks to lack of funds :-/ Anyway, any feedback would be greatly appreciated, thanks!