What will cause my truck to fall flat on it's face while going up a long incline. It seems to run fine under normal conditions but when hauling a load or lately without a load it will start to shutter and won't take any fuel. I can put it in neutral and it will rev or I can just let off the gas for about 10 secs and then I can take off again. I have replaced the fuel pump within the year and it idles at 52 lbs and will climb to 60 under load. I replaced the head gaskets last year and the spider injection unit 3 months ago. I also replaced the cats last month. Nothing has touched the issue. I have no check engine light and nothing is showing up on the scanner. I'm lost!