240sx missing, backfiring, losing power

[Satan]

My 240sx has around 260k on the body and 80k on the motor block itself. The car makes a loud whistling noise that sounds like its coming from the trunk. It only whistles if I give it half throttle or more. It only back fires if I let off the throttle while going down the road. My bumper is really dirty from the exhaust. It's covered in black dusty grime. I really don't know what to do, I've replaced the spark plugs and I got new spark plug wires. When I pulled the old plugs they didn't look bad. They only had a fine white residue on the tip of the plug. My car dies at least once a week, whenever it dies it feels like the car isn't getting gas even if I gun it ill get no response from the engine. And sometimes if I turn it on it'll start to idle at around 200-500 and I can give the car gas but it'll only rev to about 3 k and won't go past it. If anyone could please give me some suggestions as to what to do, I'm in desperate need of help.

-Satan

[Mikester]

Check & clean your MAFS. When it malfunctions, the car goes into limp mode (can't rev past 2,500rpm... runs like crap)... Also check that the connector is on good.

Verify you gapped the new plugs correctly. Also, since you did plugs & wires, you should do cap/rotor too if it's a KA.

[Satan]

I plan on doing the cap and rotor after Christmas and I already cleaned the MAF with mass airflow cleaner. It ran a little better that day but it still dies. The dying is my biggest issue.