I had similar symptoms and it turned out to be the fuel pump.  Are
you sure the Fuel pump you tried was a known working good one?

Some fuel pumps work fine for a few minutes, then deteriorate as they get warm.

>We have checked everything and still cannot find the problem. The brain and
>air intake sensor have been swapped with a great running van. Brain checked
>normal with all the tests as well. CO has been measured and hooked up as well
>and tested fine, too. Fuel pressure is fine, injectors firing well, gas is
>new and i have even swapped the pump as well... exhaust is clear and not
>clogged, throttle switch works fine...