Compress,

There might be an air leak in the fuel line from the tank. As the engine gets higher above the tank, it takes more suction to get the fuel to the engine and more air will leak into the fuel line.

Full the fuel tank all the way and try accelerating up hill. Also look under the car for any fuel leaks in the fuel line. Having the tank full will cause any leaks to leak more and easier to find.

Just because the engine revs up in nuetral, doesn't mean it has a lot of power. It takes very little power to rev up an engine under no load.

P E H