Engine starts then dies?

Hi,
I apologize for the long post, but I want to be very clear about what's going on. I'm at my wit's end with this.

This car sat up for about 2 years. We replaced the Camshaft position sensor and changed the spark-plugs, and it ran. But acceleration was VERY bad.

There were times that the car would not get over 50. We could feel in the the vibrations of the car that it just wasn't "catching, " there was sort of a "sweet spot" on the accelerator that we had to find to get it to accelerate-flooring it did no good. It would even kill the engine.

During this, the RPMs would refuse to rise-they were not maxing out as I would expect from a transmission. The tranny was shifting, the engine just wasn't getting any power.

After a while of driving it would usually start "catching, " although a speed over about 60 mph was always difficult to maintain. It just seemed as though it was not getting enough fuel. Occasionally we'd be driving down the interstate and the car would shudder and lose acceleration. Generally this could be stopped by letting up on the gas for a second. But then we were back to SLOWLY accelerating.

The service engine light was on, returning an error code which someone interpreted to mean "getting too much air to fuel." I'm kicking myself for not getting the code.

So, we changed the entire fuel pump assembly along with filter and electrical harness with Nissan parts from the dealership.

No change, but still drivable.

Took it back and it was returning codes p0300, p0731, p0732.

After researching I believe that these errors are symptoms of a bigger problem-not the actual problem. The transmission is triggering because the car is not accelerating properly. The misfire is part of the problem. But is not the problem. From what I read the transmission errors pop up a lot on Nissans when their air/fuel ratio is bad.

The Mass Airflow sensor was mentioned a lot when we were researhing-we cleaned it, and the throttle body while we were at it.

After that. The car starts, then dies. We've cleaned both of these things on other cars, and never killed them.

It still seems like it's not getting enough fuel. If we pump the gas pedal it will rev up. Up to 5000 rpms if we keep at it. As soon as we are off the pedal, it's dead. It even dies sometimes when we are on the pedal. It's not drivable. It seems like, if it was getting too much air for fuel. Now it's getting WAY too much.

We've used fuel injector cleaner several times. We introduced Seafoam into the air and fuel systems. No change.

I sure wish this car had a carb, it would help to see how it did with fuel introduced more directly.

We're thinking it's possibly a vacuum problem, the fuel injectors, the engine coils (81 bucks each!) Or the o2 sensor. We really don't want to get into "trial and error" because we've already spent WAY more money than we intended on this car.

Thank you so much for your help!