Again, keep it simple.

Fuel, Air, Spark...more or less.

Check 12v power to the ignition circuit during this period.

Check 12v power to the injector circuit during this period.

You say you've already ruled out fuel supply in terms of the pump. So check the other areas. Also, when it starts to die, are injectors pulse widths changing ? Or does it look like they are still trying to stay running ?