My truck sat for nearly 3 years when I decided it was time to get it running again. It fired up when the tow truck pulled up but ran like crap. Had it taken to a mechanic to do some work and he found the #6 injector was not working. Ran smoother then when it was running before I parked it. You can overcome a bad injector with high rpms. I'd check that.
Oh, and don't ask me how because I don't know.