I specialize in the 6.2 / 6.5 's .... if it had a bad injection pump it wouldn't run at all. sounds like there is a few different issues with that truck. Hard starting is a glow plug issue .... 9 times out of ten.
when it runs like garbage... have to see what color smoke.. Id bet a head gasket issue. may be a bad lift pump- starving the injection pump for fuel. There could be a few combinations of issues that are causing that truck problems.