It sounds like to me it is sucking air in the fuel system somewhere. Check the hard metal lines going to the filter housing and the injector pump, the brass fittings have rubber gaskets in them that after time will get hard and crack. There is also a possibility the filter housing could be cracked and sucking air. After you address the sucking air problem and the truck still seems slow with no power, change the fuel filter, if that does not help change the injection pump and injectors. Good luck!
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