Does your "Wait To Start" light come on or stay on very long when you first turn the key on? If so then I'd say it could be a glow plug issue. That or it could be a fuel supply issue. Just a few things that come to mind.
Sounds like an "air in" leak. Check the fuel filter head for leaks and the filter itself to see if its leaking and the water in fuel sensor and bleeder screw on the filter head. My truck was getting hard to start and when I did some looking it was my bleed screw, the head was almost all the way snapped off. Just a couple places to look that helped one of my problems.